just another piece of the puzzle, wouldn't say it is 100%, but it is a good place to start, IMHO. Of course, it depends on age and health of fish. Even experienced observers can get fooled occasionally. I have, many times. Watching a female lay eggs is usually a good way to decide the females, but the males can get tricky. I have had at least a few female/female 'pairs' that I didn't discover for some time.