To answer your question...yes a dentist or a hygienist can tell the difference between stain from smoking and stain from coffee or coke. I hate to disagree with my hygienist colleague, but yes, dark sodas stain teeth. Blueberry pie if you ate it every day, will stain your teeth. Stain is stain. You will get stain in all the nooks and crannies of your teeth. And you build stain around the gumline.
Smokers will for one thing smell of smoke. And they have tar stain on the tongue and gums.
However, a pack and a half is NOT going to stain your teeth.
Seeing a hygienist every six months may not keep your teeth from developing a stain that can not be removed.
Let's say you smoke. Your flossing is great, and you leave black stain around your gumline. At this point the stain is called Extrinsic stain. The stain is just on the outside of the tooth, and yes, it can be easily removed . However, if that stain stays around your gums. And every day, your teeth go through cycles of demineralization and remineralization. As your tooth remineralizes, that stain becomes incorporated into the enamel of your teeth. At this point the stain is called Intrinsic teeth. It can no longer easily be removed from your teeth. The hygienist can't scrap it away, or use an ultrasonic to bust it up. it is now right into your enamel. The only way it can be removed is by a drill.