The info that meegosh posted is about as good of a description as you're probably going to find. It's usually very easy to tell them apart -- the males' tail fins will be noticeably larger than the females. IMO, the females don't even look good at all due to their lacking tail fin features and just how small they are in general.
Most LFS will have them labeled accordingly so that you know which ones are males and which are females. Once you see one of each you will never have difficulty telling them apart again since it is such a big difference between the two.