1. He's done it before. Past behavior is, hands down, the best indicator of future behavior. Find out if he messed around during his previous relationships and if he did, beware. If he's in a relationship now and you're the one he's cheating with, don't expect him to be faithful to you either. It never ceases to amaze me that some women delude themselves into thinking that "it will be different with me." Well, it could be. Just don't count on it too heavily. Related: he has lots of exes that keep showing up, or worse, he has lots of exes with whom he has kids. We're talking multiples here.
2. He lies to other people. Have you seen him lie to others to get what he wants? Okay. We all lie about some things some times. We've all called in sick at some point because there was something else we wanted to do, for instance. But if he makes a habit of being lose with the truth, look out. If you've caught him lying to you, ask yourself what else he may be hiding. People who lie casually, and often aren't the best ones to trust with your heart.
3. There's an old Spanish proverb that says something to the effect of: "show me who your friends are and I'll show you who you are." Does he hang around with a bunch of players? Guess what. That may be a very good indication of just whom he really is. Also related, your friends and family have bad suspicions about him. This is often a very bad sign.
4. You've been seeing him for more than a few weeks and he's not introduced you to any friends or family. If this is the case, he's hiding something. Related: his friends or family hint to you that he isn't a one woman guy. They know him. Pay attention.
5. He only has particular, very rigid, scheduled times when he can see you, doesn't want you dropping by, or is paranoid about you seeing his cell phone. These are all facets of the same behavior: secretiveness. If he's being secretive, it's because he's got some secrets. Think about it.
There may be more that others could name, but these items would set off my alarm bells instantly. The truth is, if you want to keep from being played for a fool, then you need to pay attention not just to what people's words are saying, but also what their actions are saying.