What are some subtle signs that a guy gives when he is interested?

Depends on if the guy is shy or not.

Shy
Looks at you, then turns away if you look at him. Then he looks at you again if he thinks you're not watching.
Tries to be around you somehow. Like if you walk somewhere, you notice that he's there "coincidentally"
Might ask close friends about you. Hoping they don't say anything to you
Might call you and hang up *wink*
Might ignore you for fear that you know he likes you (weird I know)
----there was a girl in HS I think liked me. I thought she was cute too. But I was REALLY shy in high school. She was pretty flirty, feeling my hair, touching me, talking to me a lot, etc. But I barely showed any emotions. Sometimes, we walked past each other and she would say Hi, and I would just ignore her =( I felt so bad afterwards cause I really wanted to say Hi back but I was just too say. I thought maybe she wasn't talking to me but to someone behind me. But when she walked past me, and I turned around, no one else was around.

Not So Shy
Maybe a little flirty
Maybe a little touchy-feely
Might just straight out ask you on a date
Smiles at you
Approaches you and talks with you

Subtle: He'll probably smile and make a few jokes.
Not so subtle: Then he'll ask for your number or a date.

This will vary with experience and confidence.