The only sure way to tell the sex of an Angel is during the spawning cycle.
You notice in the second of the 2 top piccies, between the pectoral and anal fins there is a tiny protrusion. This is called a papilla. Females have a larger, more round-ended papilla, and males a smaller sharper papilla (like the tip of a pencil).
Unfortunately for us, the papillas only display for a couple of days when a pair is preparing to spawn. So you cannot really sex an Angel until its actually already spawning!
The head-bump method is okay, but unfortunately is not 100% accurate. Some males do not develop this bump, and sometimes females can look more like males.
The best thing you can do to ensure a breeding pair is buy 4 to 6 Angels. with 6, you'll have around a 97% chance of at least 1 pair. With 4: around 70%, and with 2 Angels about 50%
Jarrad