Fire is 100% a color mutation, pattern varies as much as it does in normal ball pythons. They're easier to pick out with a good picture and there should be no doubt whatsoever when shown next to a normal. Ask for pictures outside when it's overcast for best results - too much sun is as bad as too much flash.
Top 3 snakes are fires of different "shades", middle snake is a normal for comparison:
Left snake is a normal next to a busy patterned fire and a more reduced fire:
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The clear belly is promising but normals can and do have clear bellies too. Fires can have some pattern on their belly as well, so.. I'm seeing a headstamp in these but that's no guarantee either. If I were buying from an unknown source I'd want to know the pairing, see pictures of the adults and a couple clutch shots.