Does he have fond childhood memories of his parents?
If he didn't feel adored as a kid, he may have difficulty forming emotional attachments now. But if he felt loved, he can likely give his heart freely.
Does he refuse to work at a job he hates?
A guy who sacrifices security to follow his passion doesn't cruise through life in the easy lane, which means he'll stick with you through thick and thin.
Can he laugh at himself?
Being able to make light of his screwups is a sure sign that he has an easygoing disposition and probably won't come down too hard on you either.
Is he still tight with old male friends?
Maintaining long-term close ties shows loyalty, and the fact that they are guys indicates that he isn't threatened by other men... or presumably, a strong female.
How does he feel about his ex-girlfriends?
If he talks about them respectfully or is still friendly with a few, it's a pretty safe bet he's a stand-up guy since he was able to sever the relationship with grace.