Can a doctor tell if you smoke while you are taking an asthma test?

Possibly: If you are talking about pulmonary function tests, the results may show the pattern consistent with chronic obstructive pulmonary disease, which is most commonly due to smoking. But anyone could smell heavy smokers.

Maybe: If you have smoked enough or long enough you may have changes in lung function due to the development of emphysema or chronic bronchitis that would show on a pulmonary function test (pft). However, pfts are not a way to screen for smoking. There are other ways to check for smoking including serum carbon monoxide levels (nonspecific) and blood or urine cotinine levels (specific).