It depends on what kind of scales a fish has, since there are 4 main types. 3 types aren't much help, but the cycloid scales of a koi are round and have the rings. Placoid, ganoid, and ctenoid scales are the other main types, but they're no good for this. ( well, ctenoids have rings, but the weird shape makes then hard to use. )
Anyway, don't even think about ripping off some scales just to see how old the fish is unless you have a VERY good reason for absolutely having to know. Curiosity is not a good enough reason to risk that kind of damage and infection.
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