At the hospital, what would show that a person is a smoker ?

Not much: Aside from your voluntary admission, not much. A Carbon Monoxide level can be drawn and might be high in smokers but doesn't always corrolate. And this is not something that is routinely drawn anyway.

Lots of things: people who smoke smell like tobacco, and it gets on their clothes and other belongings as well. They may have tobacco stains on fingers, and tobacco smell on their breath and even in their hair. Very simple urine or blood tests show the presence of nicotine or its metabolite cotinine.