You could ask if he is a cop, but a cop would probably still be able to lie about it. It also depends where you live, if you are in an area where it is weed friendly you prob have nothing to worry about. If you live where I live, you need to extend a little caution. Chances are however he isn't a cop or narc.

You have to use your own judgement though. How well do you know him. I don't like buying from people I'm not familar with for several reasons. And I avoid any dealer I know who sells meth, crack, etc.