Some people just can't be with only one person. You can't always tell who these people are, but there are some signs to watch for. Some are useful to find a one night stand, other are good indicators to avoid these people completely. You can't always tell if you are compatible or not, but there too are signs for that.
Want to know how loyal that married woman is? Just look at her hand. The more expensive and ornate her ring is, the more likely she is to be having some fun on the side. A woman with a simple ring with a small stone, she really love the man she is with, and wouldn't do anything to hurt him. Loyalty here has an inverse relationship to the quality of ring.
How do you know if a man cheated? Did he travel alone? If he did, then he did. Men aren't generally looking to cheat, but aren't going to turn down a free meal. Don't like that idea? Sorry, reality doesn't care what you like.
On the other hand there is just crazy. Crazy take many forms, but it is never good in the long run. Now there are levels, and everyone has a little crazy to them. When you find the big crazy, just run. You can't fix crazy. Somethings just can't be fixed.
Speaking of "fixing" people. If are a broken person, and someone wants to fix you, it won't work. They will try, but you are what you are, and the relationship will end badly. So if you think you need to fix the person you are with, just walk away. Now if you are broken, just find a person with another kind of brokenness, something that complements your own, you will lean on one another and make something new out of the broken bits. When the right two broken people get together, they make a new and better whole.
One more thing, don't sit thinking about the one that got away. They aren't thinking of you, and you don't need the grief. Find someone else, even if only for a night.