This is always a tough question to answer, even in real life. A good player can fool almost everybody, but lie PT Barnum once said, "You can't fool alll of the people alll of the time." My best advice is to bring in your friends. If they do not like him, or he does not want you around them, avoid him. that is the one acid test I would rely on. Your friends could be wrong about him, but if they all feel the same way, watch out. And even if only one of them thinks you are wrong, find out if they are right.