Search "Green Terror Sexing" under "Cichlids: Central & South American".
There's a lot of good info that has already been posted.
On top of males developing the bump on the foreheads, males tend to have trailing dorsal fins, while females have rounded shorted fins(for the most part?), males are more colorful as they get older(depending on genes).
Again, do the search, you'd be surprised to what people have already posted about this.(I just started a thread about this also )