A shy guy will shy away from you more. A guy that is not into you won't shy away at all her will just stay there. A shy guy will fidget somewhat. A guy who is not into you might just stare off into space when you talk. If you go to touch a shy guy he will act even more shy and move away some what. A guy that is not into you will just boringly sit and take it.
Lead him to a private place, grab his hand, and tell him you have a surprise for him if he closes his eyes. Now kiss him gently and look at his reaction.
Voila -- you will most likely quickly find out if he likes you or not. Also you have some chance where even if he never liked you that way, he might after the kiss.
If you want to figure out what a person feels about you, and especially in a tough case like a shy person who is almost impossible to read, best to do it through action rather than through conjectures of what they might be thinking.
I don't believe there is a such thing as a shy guy. Only time a guy would be shy/nervous is if he approaches a girl he likes and he's literally on hard and sweating bullets because he doesn't know how the conversation is going to turn out. On the other hand if he's not into you then basically you'll see that he's not into you and you'll have the gut feeling also. If you have to ask yourself all the time "is that he's just isn't into me?" then 10/10 you're right ! he's not that into you as you thought he would be. A guy that is into you will give you his undivided attention and you would feel that his time is evolved around you. He can be busy and you know he is but he just wants to call you or text you, makes time for you, ask you out on dates, always wants to hear your voice etc. so many things. But you will also feel the interest in the atmosphere and you won't have to question anything about what he does towards you