Yes, it's very hard to tell the sex of Angelfish until you see them spawn. Older male Angels can get humps on their head. Some people say they can tell the sex of the Angelfish by looking at how much its body curves. However this hasnt been proved.
It is difficult, especialy when they are young. On mature angels the female tends to be concave in the area below the pectoral fins and twards the front. The males tend to be flat. This is far from a sure thing . Don T.
The only definite way to sex angels is to look at their breeding tubes when they're spawning. The female's will be rounder and fatter and the male's will be thinner and pointed.
I have three LARGE male angels and one female. Bought them all at the same time so you could not tell the sex. Now, four years later I have learned. The males are bigger, more aggressive, and have humps on their heads. All you have to do is watch them and see who fights. That's how I knew who was who.