1. You can identify with the character.
2. The character has clear virtues and flaws. (This is usually why you can identify with them!).
3. The character makes mistakes and, usually, learns from them. Often they make the same mistakes more than once before they learn from them, but then so do real people!
4. Their behaviour and attitudes make sense, given the way the writer has portrayed them.
5. You can imagine how this character would live outside of the constraints of the story.
6. They interact with other characters in a way that makes sense, even if the interaction itself isn't good. (This could be because the character really does not get on well with others or the other characters aren't great at "people skills").
7. The character has feelings, tastes in music, food etc so you would feel they "could be" a real person if somehow characters could come to life.
8. The character has emotional depth. Basically this means the reader can see if the character is shallow or is capable of more complex emotions and attitudes. Shallow characters can be appropriate to a story. It's just their emotional depth isn't very deep!
9. You can't imagine the story without them. (Always a good sign).
10. The character has real struggles and difficulties to overcome and finds different ways of overcoming them. (Unless they are a shallow creation, they don't give up at the first hurdle).