So far as I know exalted or in Creation at large you cannot ID slayer's mark nor fiend's or others. To you they look just weird symbols which holds no meaning. To get info you would need ask demon about it :P. But consider effects of his anima which is same as injured disappearances of the demons and also how his siamhata looks demonic and he himself glow in Ligriel's colors you could estimate with decent roll to make a guess I could be a powerful demon blooded heroic mortal with awakened essence and the brand in forehead may be somesort cursed brand mark from awakening my essence.<br/><br/>But overall you would not able ID infernal as it is without further information source. They are brand new thing as are abyssals.