How do i know if my cat is in pain?

Last week I just found out that my cat has a tumor and it has deteriorated half of his jaw. The surgery to fix this is a couple grand and my parents just don't have the money. The vet recommended that we put him down because he said that he's most likely in a lot of pain. I've had my cat, Sammy, since I was seven and I'm now sixteen. He sleeps with me every night and I love him to death. I would hate to put him down especially since he's not that old. He doesn't seem like he's in pain, but i'm not exactly sure how to tell. I know that cats are good at hiding pain, but I think I would notice something different in his behavior. He can still eat, clean himself, and do just about everything fine. So how do i know when the right time is to put him down? I'm not sure what to do.