You can't be 100% certain until you have a viable spawn. Two females have been known to pair & spawn. You can be pretty close to 100% when the do spawn though, females pairing & spawning is pretty rare.
Anything smaller than quarter body size is straight guessing. Any fewer than about 8 fish grown out from the same spawn is guessing as well. Generally, as angels from the same spawn approach half dollar body size, the males will be a bit larger, as well as more aggressive. They will be the first, usually, to claim territory.
The females will be a bit smaller, due to the body producing eggs. They will usually tend to stay in a small, loosely defined group, and be less aggressive. Females may be a bit wider on the bottom, while males may be a bit more narrow.
I've seen plenty of large aggressive females, as well as smaller chunky males. The bump on the head can grow on either sex, but is more common in large mature males, 2+ years old.
Much of it is observing group interaction within a well settled in environment. This is the opposite of a typical shop tank, more of the thing you will see in a larger home tank after growing out smaller fish. With these observations, you can sex at about 80% accuracy. Guessing is 50% accuracy, not a huge advantage, but not one to discount.
