Nice guy but poor oral hygiene and yellow teeth. How do I tell if he smokes?

You'll smell it. I can usually scent a smoker upon entering the exam room. Or just ask him if he smokes, and why doesn't he take care of his teeth. Poor oral hygienevtells a huge amount about a person, as well as is associated with a shorter lifespan.
Should be easy. Poor oral hygiene and yellow teeth may be common among smokers, but one probably doesn't have to be sherlock holmes to determine whether someone smokes. The smell is unmistakable, and almost imposible to camouflage. It is present on clothes, in hair, in their car more than likely, and probably in their home. Of course, if all else fails, you could always ask.