There are only two true ways to determine the gender of an angelfish, and one requires dissection.
The other way is to watch them prior to or during spawning. Both males and females have a reproductive organ called a papilla, located in front of the anal fin. In the female, it will be much more noticable, larger, and blunt. On the males, it's harder to see, thinner, and more pointed. Also, females become gravid - that is, they fill with eggs, causing them to bulge out in a nicely rounded shape, noticable if you're viewing the fish head on or from above.
Some people say that the males have a rounded crown or the females are straight from the tip of the dorsal fin to the lips, but I've had breeders tell me that this is a myth.
Also, if you have lots of angels in a tank together, the males will often all display their papilla to try and gain the attention of a gravid female. The fish who don't participate in this mating game are usually females.