I'm just curious why you are interested in knowing this. Because as a regular guy who carries, I don't really care if other normal people are carrying or not, it is our right, and more power to other CCWers.
If you are some kind of LE or govt. and want other people's opinions on how to better perform your duties, then I would say specifically spotting a weapon is secondary to just looking for suspicious behavior in general. Actually, I would go further and say that kind of applies as a "regular" citizen as well.
I would be more interested in looking out for suspicious behavior, or a person that is "out of place" than someone who I see is printing when he is reaching for corn flakes, or standing in line, etc. I can't say exactly all of what would be considered suspicious as opposed to someone just conducting normal business, but I'm guessing being really fidgety, walking back and forth in the store, waiting customers out (to leave), pretending to shop, etc. would alarm me more than anyone who I know has a gun on their person.