The only 100% guaranteed way to sex angels is to wait until they spawn. Some people take an educated guess by looking at things like a hump on males heads. I've kept angels over 30 years and I just wait for them to pair up and spawn. Angels mature at around 8 months and can spawn anytime after. When they are about to spawn their breeding tubes drop. The males looks like a sharpened pencil tip and the females look like a blunt eraser tip and is very easy to see.