Overheard a conversation on the bus - a lady was talking about giving up and said to a guy "you look like you used to smoke"

Something about baggy eyes. Hmmmm
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"There comes a time when you look into the mirror and you realize that what you see is all that you'll ever be. Then you accept it... or you kill yourself. Or you stop looking into mirrors."
- Londo Mollari, Babylon 5

I dont think so, but maybe after spending time around them for a bit maybe.

Depends on how long the person is smoking, as well as signs of long term effects like yellow teeth, yellow fingers, and coughing up the odd bit of lung tissue, there are bodylanguage signs as well. They mightn't react when third person smoke comes their way or they might check out somone who's about to light up instinctively. Actors who don't smoke, and need to smoke for a part are notorious for getting the body language wrong. From the way they are just holding the prop cigarette most smokers can tell that the performer is not one of them. There would be a hightend awareness for a non smoker around smoke and they wouldn't have the same ease, grace or confidence holding onto a cigarette like a smoker.

i used to smoke up until 8 years ago, ive gone completely the opposite direction now, bit of a exercise fanatic. i hate it when i breath other peoples smoke, i can smell it from quite far away. they do say that smoking ages your complexion, but i doubt id get a good score on a test to identify a smoker from a picture. btw i have nothing against smokers.

I couldn't tell a smoker by looking at them, but I can smell it. That's plenty to put me off.