Some early warning signs that set off my radar:
He (or she) seems to go "from zero to sixty," emotionally, and particularly regarding anger--can switch tremendous anger on and off really fast and seemingly without warning.
Punches walls or other inanimate objects, or tells stories about doing so. Particularly if s/he seems to think that this is a normal thing to do, or that s/he was justified in doing so, or especially, if s/he gives the impression that s/he is to be commended for his/her restraint and self-control (that is, s/he REALLY wanted to hit a person, the one who was supposedly provoking him/her).
Seems incapable of being angry and remembering more positive feelings about target of anger at the same time. More specifically; when s/he's in a rage at you, you feel as though s/he is looking at you "with the eyes of a stranger."
Seems to be under the impression that his/her jealous feelings entitle him/her to become a prosecutor/stalker/interrogator. You feel as though you are "guilty until proven innocent." And you feel like you're under constant parole.
S/he tells stories about manipulating or tricking other people, with no signs of guilt, maybe even glee. S/he seems to be of the belief that everyone is a manipulator, and that everyone would lie, steal, cheat, maybe even hurt or kill, if it weren't for laws or social conventions. At the same time, may try to draw you in as "special," the only one s/he really can be honest with.
S/he may have a terrific sense of humor (or not); but woe to the person who laughs at him/her.
S/he may have a slightly...*off* feel when dealing with or talking to or about somebody else who's in pain. That is, unless they're really unsocialized, a lot of these people will have learned the socially correct responses ("oh, I'm sorry," whatever); but sometimes they'll say completely off-key things. You get the impression that they know the words but not the music. That they don't really give a damn, in other words; and if the mask slips, they might seem bored, irritated, or even sadistically amused.
S/he seems to talk about revenge an awful lot.
And this last may not mean much by itself, but coupled with any of the others especially it'd worry me, and at least would give me pause: enjoys ultra-violent movies, sports, video games, and so on. Particularly if s/he talks about them at great length and with great animation and relish for the gory details, sometimes oblivious to context (at the dinner table with people s/he doesn't know very well, for instance).