there is many an arguement based on this topic.
males:
-should be more agressive and pretty much cannot stand another male in the tank
- should have a larger anal fin than dorsal
- should be more streamlined than the female
- will have a thorn-like thing hanging out from the vent when breeding.
females:
- should be less agressive and can normally live happily with another female, may even lay eggs without a male there. will (in most cases) be the victim of a male on female to the death fight.
- should have a larger dorsal fin than anal
- should be rounder in the belly
- should have a soft bulge where the vent is constantly
- will have a volcano shaped egg tube hanging out from the vent during spawning
unfortunately, other than the thorn and the egg tube, this can all be useless info as they are extremely hard to tell apart.
my larger oscar has the soft bulge, follows the smaller like a puppy and has a larger dorsal.
the smaller has a very flat vent area, larger anal fin and is very brave, likes to lead her around and explore.
the larger has been known to feed one of the 4 RZ juvies i put in there as feeders, which is sorta maternal in a species confused way lol.
the smaller has a LOT of issues with my brothers, will refuse the food they offer and tries to attack them. but me (im female - pretty sure anyway ;P) they are fine with.
there is just no way to define it - its luck and chance unless you buy a proven breeding pair.
do u have oscars? what tank? i hope this has helped a bit
xx