The only way I know of to sex angels is to check out their spawning tubes, which aren't visible until the fish are fully mature.
If you look carefully, just behind the "feeler" fins, you should see a small tube-like appendage. On females, it's usually round and cylindrical; on males, it usually has a pointed shape. This isn't a fool-proof method however.
As far as aggressiveness goes, if you have artificial plants and decor, you could try rearranging them just before lights out. If you have a planted tank, try removing your aggressive fish to another tank for a few days, then putting it back. This may change the tank dynamics, and a new hierarchy may develop, in which another fish (hopefully one less prone to bullying) may emerge as dominant.