How do you know when?

You could tell him you are on your period and will not do anything! If he still wants to go out even though he is sure nothing will happen, you can feel better about his intentions. Seriously, it works. Or you could introduce a similar situation. Telling him a friend of yours met a guy and all he wanted was a hook up. His reaction will tell you what's on his mind, if he tries too hard to convince you he wouldn't do that you can bet that's what he would do!

"Want another long island?"

He usually tells you, especially if you ask him. But what do I know, I'm Just A Random Guy.