+1 alLexX. That's true. The only way to tell is when they pair off. When they are mature enough and they are in pairs, you will see a breeding organ on the bottom that the female uses to release her eggs on something, ex. piece of driftwood. The male then runs over the eggs to fertelize them or something, not sure. Those organs are called papillas, you will tell the difference because a female has a big round papilla while the male has a long skinny papilla. Hope this helped!
As for the Leopoldi's Angelfish, I never knew they got agressive! I have read online that they are fine as a community fish, experience must be much different! Hmm...