For why Slayer MIGHT have known, is the Mark given to Forsaken by the DO. We can presume that this mark cannot easily be taken away, or Moghedien would surely have lost it as well.

We also have some reason to believe that Slayer has some kind of Mark that lets him control Darkhounds, so it's fairly reasonable to suppose that he would also be able to recognise Asmodean by the marking, even if he had never seen Asmodean before or if Asmo was disguised with Illusion.