You have to remember that straight guessing is 50% accuracy. If you have been working with a particular strain for a while you can get better with guessing, but nothing is 100%. Much of it is an instinctual thing, much is based on observing behavior of a group of acclimated & settled in pre-adults or adults.
With older angels, the humpier the head the more likely it is a male. Males are generally skinnier lower down, where females are fatter. Males tend to have an anal fin that is more vertical on the leading edge. In a group of maturing angels the males tend to be a bit larger, as the female's energy is being routed to producing eggs. Males will tend to be more aggressive as well. One trick is to take a known male, and add him to a group, observing how the other fish react. The fish that act aggressively towards the newcomer are usually males.
These are just trends, you can have a fatter male with an angled anal fin, or a large humped aggressive female. Using this, observing behavior, and gut instinct you can get closer to 75% accuracy, which is about two out of three, not really accurate at all.
Since much is based on observing behavior, sexing via the internet using pictures is 100% accurate 50% of the time.