male paradise fish have a lot more color than females, males will either have alot of blue w/red stripes or alot of red w/blue stripes. depending on if its a regular or blue paradise fish. the fins of a male grow much longer than the females as well.
Look at the tail, imn the male, the tail will have two filaments syicking out from the sides like a lyre tail molly....depending on age, the tail is usually half the size of the body. Females tail is rounded compared to the males.