Well... you look. Bearded males will have a nice little triangle consisting of the beard and ear muffs. Ideally they should be oval shaped.
Not my photos, but here are some examples:
Bearded:
Non-bearded:
As for genetics, muff/beard is dominant, so a bearded bird crossed with a non-bearded bird would produce 100% bearded birds, however the beards would be smaller than in a bird pure for the gene. A small beard bird back to a full beard bird would produce 50% small beard and 50% full beard offspring, and a small beard bird to a full beard bird would produce 50% small beard and 50% non-bearded offspring.