I think my dad smokes weed?

Ok, so my parents got a divorce over these past couple months. (I'm 15) This week, (we switch every week) I'm staying with my dad. He and I have bedrooms RIGHT next to each other. Tonight I went out into the hallway to go to the restroom and I smelled a very unique smell coming from his room. I've smelled this smell around him many times before but for some reason it just registered. It smells like pot!! His cologne has a similar smell but this was much stronger and a little more bitter then his cologne. I heard from a family member that he used to smoke weed but i dont know if that is true. It wouldnt surprise me if he did because he use to follow the Grateful Dead and was kinda a hippie. I dont have a problem with pot even though ive never smoked it. But he was living in the basement until he moved out and his room smelled the same way. Only HE smelled like it before he was in the basement, the smell was never anywhere in the house so he may have been smoking outside to hide it from me. How should I approach him to find out if he smokes weed? I was thinking of saying to him I smelled something coming from his room last night and it smelled like pot. What do you think of my plan? and other suggestions? Thanks!