I would think that a person sufferimg from megative levels would appear weak, slow and uncoordinated, probably having trouble focusing and following conversations. It also seems reasonable that depending on the source of the negative levels blackened or bleached out wounds might show. I know of no specific text to support anymof this, it just sounds like good description and immersion material to me.
I think Java Man is on the right track.
Also, I believe a character's perception of a level-drained character would depend on their familiarity with the victim before the trauma took place. A GM going for a more subtle approach might describe it as a vague sense of seeming somehow "diminished".
Negative levels are for chumps. Real undead drain Xp.
Negative levels are for chumps. Real undead drain Xp.
I was attacked by a vampire and now I've retroactively never attended college.
It is up to the DM how such people look to others. I am with Java Man's here, probably adding something small to hint at the source of the problem (a dim red spark in the eyes, less shadow than there should be, maybe an overlong canine).