I'm just curious. But how do you act if you like someone? And how do you know that someone likes you?
There's some physical signs in context. Constantly worrying about your safety or wanting to make sure you "stick around" is a good sign someone is into you even if they haven't said anything. Also, you don't want them shying away from taboo topics with you. Not shying away doesn't necessarily mean anything, but completely avoiding them does. If they don't want to "get personal" with you and keep most discussion to small talk, I'd say they are not truly interested.
Touching is another one unless they're like that with everyone or most everyone. Usually though if the touching seems like it's pretending to be inadvertent, like constantly brushing against you, holding and rubbing your arm, wanting to be way in your physical space, then it means something. Something very overtly flirtatious, particularly when the girl is trying to be very sexy all the time, is usually bullsh**.
It's in fact the same bullsh** I do all the time.
I talk in this very even tone that sounds an awful lot like Ryan Gosling from
Drive. I kind of adopt this Downey Jr. sense of humor. That's all premeditated. It's for entertainment purposes only, but that's not to say I don't know it can get me what I want without actually having to commit to anything.
The more incidental touching tends to mean something.
Because honestly listen to all the guys in here "I like this person but I don't know how to say so...".
Think anyone is really that different?
More than likely any girl with some actual interest in you, whether it's an intense sexual fascination, or something else/more, isn't going to want to get shot down either. So you have to consider real blunt signs like grabbing your junk and saying "mine" are more than likely not going to happen.
Also, another good body language indicator is staring at you, perpetually, kind of to the point where you get creeped out. When they have a deer in the headlights look, like it hasn't occured to them they are staring. There's almost no two ways to read that (unless you have something on your face). Usually that's just plain sexual. Most women/people are not aware when they do those things.
Getting nervous around you is another good sign.
Most of what I look at comes from remember what it was like for me when I "liked" someone. I think about the things I did or how I behaved and then see if women I know exibit those same signs. Sure enough there has been a lot of cross over. Some of my game (although I think game is bullsh**; separate discussion) comes from mimicking girls who I was into, except doing it in a "guy" way.
People have their own speeds though. The signs may be 'there' but your brain realizes what it wants before you are consciously aware of it, so you can't get all Rapey McRaperstein on them just because they flash you the eyes. More often than not as long as you are getting signs and those signs are getting more intense simply means that eventually, as long as you keep asking them to do things with you, they'll just eventually go there.
Probably depends on where you are. In Los Angeles a few hours is what many people call their daily commute. 
