I had 3 pairs of Rainbows, and I found it very difficult to sex them whenever I needed to . . . when they are relaxed and pairing, it can be easy to tell, but when they are stressed for any reason, they suddenly all look the same (!)
generally speaking, the males are larger, more colorful, more aggressive and have longer finnage (nothing new there)
by the way, when I say "more aggressive", that is a relative term; IME, these are some of the least aggressive cichlids I have ever owned . . . they make great tankmates . . .