Many thanks to metjm for providing me with the links to his full database of marble fades. With his help I've produced a full ranking of the marble fade pattern as it shifts from 1st max fire & ice (pattern index 412) to what is considered max fake. I produced the below ranking using karambits as they have the largest set of possible fire and ice patterns and are the easiest to judge each pattern shift.

How to check if a knife is Fire & Ice:

1. Inspect the knife ingame
2. Hover over the "Item Certificate" on the bottom right of the inspection window
3. ctrl + f the "Pattern Template" number to find it's ranking in the list below. 

If it is listed, check the column for your knife type (Karambit / Bayonet / Flip / Gut) and check what max F&I it is or if it is a fake. If your pattern index isn't listed then it is not a true F&I. If you want to check if it is a fake or not please use the rest of the guide below (not all fake pattern indexes are listed).

New Marble Fade finishes have just been added in the Spectrum Case. It has been confirmed that fire & ice patterns do not occur on the new knife finishes. 

Fire & Ice patterns are possible for:

• Karambit
  
• Bayonet
  
• Flip
  
• Gut

Fire & Ice patterns are not possible for:

• M9 Bayonet
  
• Butterfly
  
• Bowie
  
• Huntsman
  
• Falchion
  
• Shaddow Daggers

The marble fade pattern for Karambit, Bayonet, Flip and Gut knives has many different possible patterns depending on the pattern index of the knife (the "pattern index" is what determines where the pattern begins or sits on the blade, not to be confused with the paint index "413" which determines what skin the knife is. 413 = marble fade).

The most desirable pattern is Fire & Ice in which the blade is 50/50 red/blue with no yellow on the play side. I've put together this guide to help discern what is a true fire & ice, and what is fake (a fire & ice is considered a fake when any yellow begins to appear on the blade).

This guide can also help determine how "max" your knife is by using the ranking at the end of the guide.

The Karambit has 10 different maxes within the first 43 pattern shifts.

Gif of first 50 pattern shifts: https://gfycat.com/ComposedImpassionedBlacknorwegianelkhound
I don't know why it blurs at the end, click on the gif and click on "HD" to remove the blur, same for all 4 gifs

1st Max - Pattern Index 412

Max Fake (11th max+ / rank 44+)

As the pattern shifts left to right yellow begins to be introduced at the tip. The karambit is much more strict than the other 3 knives (not a bad idea as there are many more possible maxes) - as soon as any hint of yellow/non-pure blue appears this is considered a max fake fire & ice. The rankings below show all pattern indexes that were available that are true fire & ice. At the end is a list of all what are considered to be max fakes and regular fakes.

The previous method of determining the max of a karambit was flawed, and lumped many pattern shifts together - which meant that two 2nd Max or two 3rd Max would be considered equal, whereas one was noticeably more max than the other. The purpose of this guide isn't to disrupt the current pricing of 2nd-4th max karambits so I have included the old maxes on the ranking table below. 

Fake F&I or Tricolour?

If too much yellow appears at the tip of the blade then it the knife would be considered a tricolour. A karambit can be considered fake fire & ice if the blue on the blade doesn't reach the corner of the handle shown in the below image.This is harder to discern compared to the other knives. Boarder line fake/tricolour are very similar in price so the differentiation isn't critical.

The Bayonet has 7 different maxes across the first 24 patterns shifts.

Gif of first 50 pattern ranks: https://gfycat.com/JitteryTheseArabianoryx

1st Max - Pattern Index 412

Max Fake (rank 25+)

It's a common misconception that the "yellow" at the tip of the blade makes these knives fake. This is infact not yellow but orange (metjm doesn't have the correct lighting to show this) and this colour is not definitive of a fake fire & ice. Ap0c has a great guide on how to determine if a bayonet is true or not so I will leave the explanation up to him:

Bayonet Marble Fade - Fire & Ice Guide 

Counter-Strike: Global Offensive 的一篇指南

作者：Ap0c

The most in-depth, comprehensive and accurate Bayonet Marble Fade - Fire&Ice guide to date.

Fake F&I or Tricolour?

As the pattern shifts yellow is introduced at the handle end of the blade. Credit to www.reddit.com/user/Unbelievabob for his guide and how to determine if a bayonet should be considered a fake fire & ice or tricolour using the following description:

...a fake as having less than about a fifth of the blade base of solid yellow. No yellow should extend beyond the start of the 'dip' in the blade, as shown by this image:
(for clarification any solid yellow present within the width of the 'dip' is considered a tricolour)

The flip has 4 different maxes across the first 11 pattern shifts.

Gif of first 50 pattern ranks: https://gfycat.com/PoliteSizzlingGreatargus

1st Max - Pattern Index 412

Max Fake (5th max+ / rank 12+)

Flip fire & ice are rare as they begin to show yellow as the base of the blade quite early. Around rank 12 there is sufficient yellow to be considered a fake.

Fake F&I or Tricolour?

A flip can be considered a fake fire & ice up until the yellow extends beyond the edge of the button as shown below: 

The Gut has 7 different maxes across the first 24 patterns shifts.

Gif of first 50 pattern ranks: https://gfycat.com/AnchoredDetailedIndianringneckparakeet

1st Max - Pattern Index 412

Max Fake (rank 25+)

The Gut starts to introduce yellow onto the hook at the same pattern index as the Bayonet. As you can see the blue sections on the blade begin to cross over as the pattern turns to fake.

Fake F&I or Tricolour?

As the pattern shifts across the blade yellow begins to be introduced at the hook of the blade. A gut can be considered a fake fire & ice so long as the yellow doesn't extend beyond the hook: 

By using metjm's database of 930 of the possible 1000 pattern indexes for the karambit, and with the 70 missing patterns provided by www.reddit.com/user/Alamer99, I've found every Fire & Ice pattern possible and ranked them in terms of their pattern shift from Max (412) to max fake.

i.e. if your pattern index is 332 then your fire & ice is the 9th best pattern possible, or 8 pattern shifts away from 412. Multiple pattern indexes in one rank means each of those pattern indexes is essentially identical (there are some miniature pattern shifts for some, but these are small enough to be negligable).

Rank	Karambit Max	Bayo/Gut Max	Flip Max	Pattern Index
1	1st	1st	1st	412
-	-	-	-	↓ Old Karambit 2nd Max ↓
2	2nd	2nd	2nd	359, 393, 602, 649, 701
3	2nd	2nd	2nd	16, 146
4	2nd	2nd	2nd	241, 541, 688
5	3rd	3rd	3rd	281, 673, 743
6	3rd	3rd	3rd	292, 344, 994
7	3rd	3rd	3rd	152, 777
-	-	-	-	↓ Old Karambit 3rd Max ↓ (may depend on the guide you are using for where 3rd starts)
8	3rd	3rd	3rd	628, 792
9	4th	4th	4th	332, 129, 918
10	4th	4th	4th	126, 787, 874, 908
11	4th	4th	4th	923
12	4th	4th	Max fake	48, 780
13	5th	5th	Max fake	204, 252, 652, 736, 832
14	5th	5th	Max fake	705, 182, 457, 660, 685
15	5th	5th	Max fake	522, 578, 988
16	6th	6th	Max fake	230, 982, 607, 773
17	6th	6th	Max fake	444, 452, 471, 631
18	6th	6th	Max fake	356
19	6th	6th	Max fake	340, 621, 761
20	6th	6th	Max fake	112, 876
21	6th	6th	Max fake	873
-	-	-	-	↓ Old Karambit 4th Max ↓ (may depend on guide you are using for where 4th starts)
22	7th	7th	Fake	14, 274, 614, 653, 795, 826
23	7th	7th	Fake	405, 454, 683, 728
24	7th	7th	Fake	8, 32, 58, 108, 233, 732, 770, 803, 867, 949
25	7th	7th	Fake	213, 243, 702
26	8th	Max Fake	Fake	337, 378
27	8th	Max fake	Fake	188, 202, 971
28	8th	Max fake	Fake	5, 178, 406, 539, 696, 854, 966
29	8th	Max fake	Fake	655
30	8th	Max fake	Fake	461
31	9th	Fake	Fake	493, 922, 959, 997
32	9th	Fake	Fake	68, 149, 171, 637
33	9th	Fake	Fake	287
34	9th	Fake	Fake	165, 206, 370, 656, 766, 817
35	9th	Fake	Fake	499, 516, 672, 706
36	9th	Fake	Fake	121
37	10th	Fake	Fake	238, 545, 591
38	10th	Fake	Fake	177, 546, 589, 977
39	10th	Fake	Fake	764, 844
40	10th	Fake	Fake	156, 868
41	10th	Fake	Fake	402
42	10th	Fake	Fake	28, 791
-	-	-	-	↓ Be warned without ingame screenshots I cannot guarantee these are true - may be revised ↓
43	10th	Fake	Fake	553, 559, 725, 810, 858, 972
44	Max fake	Fake	Fake	372, 441, 632
45	Max fake	Fake	Fake	329
46	Max fake	Fake	Fake	232, 756
47	Max fake	Fake	Fake	727, 753, 976
48	Max fake	Fake	Fake	351, 809, 941
49	Max fake	Fake	Fake	90, 785
50	Max fake	Fake	Fake	805
51	Max fake	Fake	Fake	195
52	Max fake	Fake	Fake	561, 590
53	Max fake	Fake	Fake	27, 459, 710, 930
54	Max fake	Fake	Fake	397, 404, 473, 483, 537, 626, 818, 962
55	Max fake	Fake	Fake	254, 909
56	Max fake	Fake	Fake	125, 869
57	Max fake	Fake	Fake	647
58	Max fake	Fake	Fake	203
59	Max fake	Fake	Fake	183
60	Max fake	Fake	Fake	9, 110, 980
61	Max fake	Fake	Fake	448, 989
62	Fake	Fake	Fake	506
63	Fake	Fake	Fake	196, 309, 674
-	-	-	-	↓ Refer to the Bayonet/Flip/Gut sections of the guide to determine if Fake or Tricolour ↓
-	Fake	-	-	3, 60, 62, 71, 170, 216, 222, 234, 266, 307, 315, 321
-	Fake	-	-	328, 333, 364, 400, 411, 413, 438, 445, 463, 485
-	Fake	-	-	496, 535, 582, 605, 624, 630, 663, 667, 670, 691
-	Fake	-	-	717, 723, 746, 794, 822, 845, 846, 931, 948, 958

This ranking was painstakingly completed by comparing images of all F&I patterns. There may be a chance of typos or my definition of max fake is not the same as yours (my focus was on all true F&I so not all fake fire & ice may be included in the list). I've tried to give benefit of doubt for those patterns right on the cusp of max fake / least true - please let me know if I have made any errors.

Some interesting stats:

From the large list of metjm screenshots (1000 different patterns analysed - thanks to www.reddit.com/user/Alamer99 for providing the last 70 missing from the database) I recorded 123 true fire & ice karambits, or about 12% of all patterns. From my findings for the other knives this is the rarity of Fire & Ice compared to other Marble Fades:

• Karambit: 12.3% (123 possible F&I patterns)
  
• Bayonet: 7.1% (71 possible F&I patterns)
  
• Flip: 2.9% (29 possible F&I patterns)
  
• Gut: 7.1% (71 possible F&I patterns)

Fire & Ice Owners Groups

http://steamcommunity.com/groups/truefireandice - For owners of any true F&I knives

http://steamcommunity.com/groups/bayofireandice - For owners of any true F&I Bayonets