HOW DO I TELL MALE FROM FEMALE??

I just picked up a pair that was supposed to be a sexed pair but they look identical to each other.. I can assume since males are territorial and would tear each other up, I do not have 2 males, however I have a 3rd animal I hope to introduce as soon as quarenteen is up..4-6 months.. I hope to captive breed these guys but I need to know if I still need to find a male or not..

any advice is welcomed...

thanks in advance

harpy