Parrots are hard or next to impossible to sex based on dimorphism. They are hybrids of a number of fish so looking at fins or the nuchal bump is no indicator.
Cichlids, in a community environment, will fight to establish a pecking order. Generally, the strongest (largest) of the group will bully the most likely competitor...another cichlid that's about the same size and will tend to leave smaller (less competition) cichlids alone.
Therefore, if your Green Terror and Parrot are about the same size and he leaves the parrot alone, it's a good chance that the parrot is a female. If the parrot is much smaller and the Terror spends most of the time chasing off a large dempsey, then I'm not so sure.
You also mentioned that the Green Terror is chasing everyone away but the parrot....that may be a good sign as well and he may be courting her. Cichlids will pair off even in a community setting....one of the key things to look for is heightened aggression, lots of digging in the gravel in a particular spot, cleaning and fanning of a rock, etc.