On a widebody you could not measure it unless you were a boffin with a very accurate computer program. On a narrow body jet you probably could, but it would not change the stab trim to any measurable degree. On a Cessna 172 it might cause a problem, but you would have to be pretty unobservant to not notice an extra or missing pax in one of those. Of more concern these days is how someone could get on board or go missing and the Captain not know about the change before takeoff. If this is happening at your airline you need to review your dispatch procedures.