(Funny vision of you rolling ink on a fishes fin, er I'll just take your prints before you go in, thank you!)
To begin with size should be different, some fish have slightly different markings, maybe 6 of my clown loach I can tell by a marking difference of some kind, one is masked one has a saddle etc etc. and as I mentioned the biggest and smallest fish are always plain enough to see. The glowlight and neon tetra are a bit more tricky so I don't try to tell them apart although there are a couple that I think I know the look of, but it's mostly a physical shape difference than a pattern difference. Some have a different look, just ever so slightly.
I find with shoals of fish the only important quick check is number of fish. Count them in, normally anything obvious shows up on the count.
All fish present, happy days.
(Alternatively, DNA, perhaps for the most fishy of fishes only! Swab 'em!