How can you tell if he's only in it for sex?

Now don't get me wrong. Sex is awesome. HOWEVER, I am currently looking for an actual relationship and am not interested in wasting my time dating a guy only to sleep with him and off he goes.

I've been single for a while now and my last relationship was a long, serious one so I'm not so versed in fending off the fuck boys.

I always figured that if you made them wait a bit for sex then the ones only interested in a "hit and run" wouldn't put that kind of time in. But apparently, according to my friends that is not the case. And apparently Fuck boys aren't easy to detect as they come across super nice and "boyfriendy". Yay dating.

Sooo any solid ways of weeding out the fuck boys from the ones actually looking for a relationship? Guys and girls opinions welcome!
How can you tell if he's only in it for sex?
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