I have been blessed with a lack of players who want to play evil characters "FOR TEH EVULS!1!" But now and again you run into them regardless. Of course there are always a few tell tale signs and for some reason the players don't want to accept that it might give them away. "Why, why does huge black feathered wings make people think I'm evil." "Why do my demon horns make people run away?" Sometimes there is no explaining. Just head shaking and sighing.
Art from here