you can get a conversation going about it, or you can fake like yout talking on the phone to your buddy about getting blazed later, buying a dimebag, or w/e. and let him hear you talking. but heres whats always worked for me..
me "so, do you smoke?"
them "no, why do you?"
me "every now and then, when its around"

why i choose to come out and say "every now and then" is because, if i ask them if they smoke, they could actually smoke but think im not cool with it, and say no. so if i say "every now and then" then it might give them the balls to come foreward and say they do smoke. and if he is actually against marijuana use, then he wouldnt really say mutch since the answer is just "every now and then"