His voice or he maybe heard of him
I don’t think so because he didn’t hear his voice, the bartender was the one who greeted him and he said “sir” even before he said a word. But maybe he heard of like you said but he also seemed surprised by his beauty which can also mean it was his first time meeting him.
He probably looked at his neck because all males have that little ball thingy idk what's it called or he probably looked at his chest bc how flat he was
Probably
How did he know the Omega was a man? Looks like a woman to me!