First of all he needs to say he loves you to you, but what will really prove how he really feels is whether or not he will tell his friends or your friends that he loves you. If he won't admit that he loves you to anyone but you, he probably isn't quite sure just yet, but keep in mind, he might avoid telling his friends he loves you because they might embarrass him. But if you can find out that he told someone else he loves you without knowing that they were going to tell you, then he really loves you.
You should also try to figure out what kind of priority you have in his life. If he sacrifices other things to be with you, that's a sign. Also, if he gives you gifts, this isn't always true, and it doesn't have to be jewelery or flowers, just something to show that he's thinking about you. If he thinks about you often enough to get you gifts semi-often, then either he loves you, or he's just trying to convince you that that's the case so that you don't leave him. I think that how often he puts you and your needs/wants before himself and his own needs/wants is as good a sign as you will get.