When looking at a pair of angels which are mated, the male will have a higher dorsal fin and is more stationary in the tank while the female is more active overall. If they aren't a pair, it is almost impossible to sex them until they are preparing to spawn.
When that happens, the male will spend considerably more time preparing the site for spawning, with the female acting as a sentry and fending off intruders. She will also forage more actively for food at this time.
In my experience, spawning takes place in the dark, although they can also spawn during the light cycle if they have the tank to themselves. Just prior to spawning, small anal protuberances will appear - the males have thinner, pointier tubes whiles the female will display a thicker and somewhat shorter one.
After breeding angels for 20+ years, I have come to recognize subtle differences in the behaviors of adults, but until then it is virtually impossible to sex them. Good luck with your pair. If they are constantly tending and hanging with each other, it's a good bet that they are pairing off.