I have been around people who smoke all my life, and so I know what the smell is like. Sometimes people sit beside me in lecture, and this smell comes from these people beside me.
However sometimes when it's raining and the weather is cold, a very similar smell comes as well.
So I can never be 100% sure what I am smelling, smoke or just the air outside. What's the best way to ask "do you smoke", without sounding too nosey or weird?