THere is not a good answer to this question, it varies so much from one group of fish to another, many fish there are no external signs that are accuarate.
In some fish it can be coloration (many african cichlids from Malawi for example) Or it can be size, shape, fins, and many more. But these differances can not be used to determine sex on all or most fish, only groups. For example in live bearers most males have elongated anal fins but on most other fish, this is not an indication of sex.
Is there a specific group of fish you have interest in? (african cichlids, S.A, C.A. cichlids, etc..) That might a little easier of a question to give some ideas on.
Joel