DrNik, I breed angels and discus as well, not quite been a year but getting close.
Every single time my pairs have spawn(angels only im speaking) I notice that the male ALWAYS is the one with the enlarged crown, where the females always have none and their nose/mouth region to eye location is always an inside point rather then a smooth slope. These are P. Scalares I am speaking of.
Ive been able to be 100% accurate on dividing my males from females when I get new additions or grow out juveniles to maturity. I know the vent is deadly accurate but is this a coincidence(in over a dozen different pairs or is this something that holds water?)
Ive kept angels for about as long as you have been breeding them and have old school sexing ways from my father and some of his society members that are still in the hobby and alive. Its just something Ive kept in the back of my mind for a while and am furthering research this as well.
Also, I have a P. Scalare "smokey" that ALWAYS has his vent out, he is a proven breeder male but he lost his mate of 3 years a few weeks back, since his first spawn it hasnt retracted and there is nothing spawning in the tank other then pangio oblongas and corydoras.
This can be fairly accurate on most domestic angels in which the base genetic stock has been domestically crossed for quite a few generations. On the more recent wilds and wild crosses this method is hit or miss, most do not have typical cichlid head shape. It can also be difficult to determine the sex by the enlarging of the abdomen as the males tend to load up with semenal fluids.