I think you begin by listening to what he says as well as what he doesn't say. Observes how he (treats) you. Does he show an interest in your day to day life or display any sincere "friend qualities". He will also ask you for your opinion on issues he dealing with. This shows he values your thought processes. After dating for about 3 months or less a guy who is serious about you will introduce you to people in his "inner circle". (best friends, co-workers, siblings, or parents). Another clue is he will talk of future (plans) such as weekend getaways, holiday, or upcoming birthday celebrations.
However when it's all said and done you can look him in the eye and ask him point blank. If he is evasive or offers answers without clarity it may be he is not where (you) want him to be at this time or it could mean he doesn't see you as possibly being "the one" for him. You have to trust your gut.