It depends on the species as far as sexing. Some have females that are a different color(dimorphic, I believe is the term), so it is easy. The ones whose females are the same color(monomorphic?), can be difficult to distinguish. Many say that males have egg spots on their anal fins, but this is not always accurate either because many females do to. If you observe enough, sometimes you may be able to tell that the males have more spots or they are more distinct. But the only real positive way of sexing in these cases is by venting. In this, you remove the fish and look at the genital area to determine gender. I personally haven't done it yet, but there are ways to tell, just do some research. What species did you wind up getting?