At your age (20) I would say don't start a relationship expecting or wanting it to be too serious, dating should be fun for both of you and to see how you get along. Things will develop if they are meant to be. If you have been dating for some time already you could ask him if it is monogamous between you, or if he wants it to be monogamous between you both, that may give you an indication of how serious he is about dating you. As others have suggested here 'What's the Rush'.
thank you very much. I really appreciate your help.
Though Duttoneer's answer is good, I'd like to add to the "what's the rush" angle and say that knowing if someone is serious does require trust, and this does take time to build. If you can truly trust each other, then you can trust that the affection the other shows you is sincere. Then, if you're both serious, you will certainly feel it.