How do you know if a man is mature? People act differently in public and around certain others than in private and around certain others.
When I say "mature", I mean bottom line maturity. A man that sticks hard to his values and convictions. (He isn't wishy washy). He can back up what he says. A big one, is how kind is he? Is he just "nice guy", or does he CARE ABOUT OTHERS.
We all have different personalities, quirks, sense of humor. Being a goofball sometimes doesn't make me immature. Knowing social cues, like, when to leave, how to greet, how to be polite, showing acts of kindness, not being OBTUSE, etc.
What is his job situation, practical thinking patterns, spiritual obligations, etc. You can tell these things within 3 months of knowing him. Intuition is a strong attribute God gave us to determine these things.