One of the best ways to size up your daughter's boyfriend is to invite him to dinner. If he really cares about her, he'll be willing to spend some time with her family. Even if he's shy or nervous about meeting everyone, he'll make this extra effort to show her that what is important to her is also important to him.
Don't view this time as a scientific experiment -- your dinner table is not a laboratory, and he is not a lab rat. Don't over-observe him or watch every move he makes. You'll have to remember that he's likely to be quite nervous. Put him, your daughter and everyone else at ease by having a lighthearted conversation over dinner. That's all you should expect during this, your first "family date" with him. Don't make him do all of the work for the conversation, either. Ask him benign questions about himself, his interests in or outside of school, what television shows or music he prefers, what movies he's seen lately -- small talk.
Naturally, there are a few things you'll want to watch out for:
• Is he respectful to your daughter? Does he shush her or say anything that makes her squirm? If so, not good.
• Does he display too much affection toward your daughter? Dad won't appreciate that.
• Does he have enough sense to avoid cursing or talking about inappropriate subjects at dinner?
• Did he choose proper attire? Chances are he won't be wearing a Motley Crue tank top and ripped jeans to dinner, but stranger things have happened.
After he leaves, talk to your daughter about him, and tell her why you liked him. Ask her what it is about him that she's drawn to. If you didn't like him, try to word it in a very cautious, non-judgmental way.
Whether you agree or disagree here, at least you're taking the time to talk about something that is important to her. By getting to know your daughter's boyfriend, you'll be getting to learn about your daughter from a whole other perspective.