like what bobysgrl said. males have long fins ( to show of fo da ladies 8) ) and the ladies got short fins.
there is an exception however: you may know of the different types of betta splendens, like crowntails, halfmoons, etc. there is a type called a Plakat or short tailed betta that can be confused as a female.
also, males make a thing called a bubble nest. they make this nest when their ready to... make love. youll be able to tell its a bubble nest because there will be a bundle of bubbles in the top corner of the fish tank.
Hope this helps 8)
try also going to bettatalk.com
pm me if you have any questions