The females are smaller than the males; that's the only real way I know of to tell the difference.

I have two spotted cories in my 10g tank, and will probably add a 3rd when I get back from vacation later this month. Cories don't carry a big bioload, so if your water conditions are really good, and you do regular water changes, you can usually squeeze another one or two into a tank without risk of serious overcrowding.
When I get my 56g going, I'm going to have 6-10 of these little delights (probably 2 schools of different types).