How do you tell a male pleco from a female pleco ?

I have 3 large pleco's and i thought maybe i would try to breed them. Can you tell the difference male vs female ?
Answers:
According to Vires about Pleco's, ''Gender is difficult to notice unless the fish are at full breeding size.

Probably the easiest way to sex these fishes is by the shape of their foreheads. Males have a flatter, more leveled slope to their foreheads and are slightly wider. Females have a more rounded forehead and are slightly plumper when in spawning condition.

There is no other noticeable difference in colors or finshapes and growth in either.''