Well.. if guys always find an excuse to touch you.. your shoulder, etc... it could be that he likes you.
Also.. eye contact maybe.
Depends on the guy. When I (being someone who UNDERSTANDS body language) is "testing" a girl to see if she "bites", I keep eye contact, and break it slowly (generally while smiling), I will engage in small talk, I'll do things to make her comfortable around me like introduce her to friends/aquantences, I will touch her if she makes me laugh etc... I give her a wink or two when appropriate... Give them my classic sh*t eating grin (haha).
I've yet to have a girl that DIDN'T know I was flirting with them when I was. Oftentimes I have girls thinking I'm flirting with them when I'm not... (I tend to be a flirty (with women) alpha male type...)
It really depends on the guy. Some guys (READ: MOST GUYS) have this ego where they "have to figure it out" on their own without spending a few weeks studying body language, flirting etc to become more attractive to themselves and others... That's fine with me, it neuters my competition.
Women usually have a very good (READ: BETTER THAN MEN'S) intuition when it comes to attraction. So trust your gut.
his must be young guy thing becasue me and my friends don't do that at all, we just go and talk to the girl.
Staring is usually a key they like SOMETHING.
Of course the most overlooked option is to ask and progress somewhere.
He'll look/stare at you often. If you try look at him back he will quickly turn his head.
His body will get a bit tense when he's near you.
He might subtly try to impress you.
He might smile at you.
It somewhat really depends on the guy. If the guy's really shy he might just stare/look at you often. If the guy's slightly more bold he might try to flirt a bit with you.