How do you know that a guy is serious when he says that he likes you?

For the last few days I have been pestering my guy friend and asking him who he likes. Yesterday on MSN he was joking around and then he says "I like you". But then thing is, I'm not sure if he's serious or not. When I say that he's lying, he tells me, "You don't even trust me?". And another thing is that whenever he says "I like you" he would always use these smilie faces. And then the next day when he visited my school, I went up to him and asked him if he would tell me who he likes, because I still think he's joking about liking me. And then he just tells me that he's going to tell me on Friday when he sees me. But he purposly chose Friday because I don't have school on that day. So how do I find out if he's serious or not?