It is nearly impossible to determine whether an angelfish is male or female when they are young. However, as they age, the males develop thicker bodies, tend to be larger than the females, and they often - but not always - have a hump on their heads.
If you are in a pet shop and you see adult angelfish, you can rest assured the females will not have a hump. The males will tend to be more assertive in a tank, and they can become quite aggressive if they are mating or defending their eggs or young.
