It is embarrassing to assume a guy wants to be in a relationship with you, when he has no desire to be. Knowing whether or not he wants to be in a relationship can be a tricky thing because guys can show interest in a girl for different reasons. A guy could be lonely and simply want someone to fill the space and time, he could just want you for sexual reasons or he may want a long term relationship.
Take note of the amount of times and when he calls. If he calls you for no reason, at various times of the day, then he is likely thinking about you because he is seriously interested. A guy who likes you wont wait several days after seeing you to make some sort of contact.
Assess how much and where he hangs out with you. When a guy wants a relationship he will want to hang out with you as much as he can. He won't care the activity, whether it's lunch, a movie, a trip to the zoo or even washing clothes together at the laundromat. Basically, he won't just want to go out late on a Saturday night.
Figure out if he makes time for you in his life. Even if he is going away on a business trip, he will want to see you before he goes, keep in contact while he is away and/or see you as soon as he gets back.
Assess whether or not he's talking about you to his friends. If he brings you around his friends and they try to be on their best behaviour, it's likely he has told them about his feelings for you. If you have never met his close friends, it's likely he isn't considering a relationship at this point.
Observe whether or not he is taking interest in hanging out with you and your friends. He will want to know about them and make a good impression with them, while reserving his affections for only you.
Take it as a sure sign if he expresses his feelings for you clearly. This is the best way. He will have the courage to say, "I really like/love you and want to be in a serious relationship with you."
If you really want to know if a guy wants a relationship and he is not telling you, ask him. It will be really awkard, but at least you will know and not waste your time wondering.
A guy may pretend to want a relationship just to be physically intimate. Make sure he gives full commitment before you invest your feelings any further.