How do you know that the guy is serious about you?

Well, you never really know the exact reason why a guy is serious with you. But some things you can see when a guy is serious about you is that he pressures you into doing things that are GOOD FOR YOU, not just fun for him. He will want you to do them because your happiness is one of the most important things to him. He might be more aggressive. He doesn't just fool around with you or joke, he starts to take it like marriage where he takes responsibility.

If you do not want to be serious with him, just sit together and talk it out. Don't be rude, be sweet, kind and totally nice.

If he gets mad, tell him to relax, it's nothing bad, you're not breaking up with him or anything. It will work out. Promise :)

==========

Keep in mind that there is a big difference between being "serious about you" and "seriously wanting to get into your knickers" - big, Big, BIG difference.