Do you give everyone you see a once over?
Nope, don't have time. Just people that catch my attention.
Do you do you a basic threat assessment on everyone you see?
Same as above.
Look into their eyes, glance at their waist area to see if there is a bulge there or an outline of a gun there?
Only those that catch my attention. (I look at people often, gets me in trouble. I can't help it, I'm a people watcher. I get a kick out of watching people's demeanor and reactions for some reason. Like my grandfather, I could grab a bowl of popcorn and be entertained by simply watching a crowded street and observing interactions. It has gotten me in trouble more than once with girlfriends, but I'm honestly just observing.)
How do you know that the average Joe in line in front of you at 7-11 is unarmed?
I don't. But if he throws up red flags, he gets my attention.
Why is a person who displays their gun openly more worthy of the extra look than the person who MIGHT have a concealed gun?
Because I'm a people watcher. The guy has a gun. He is either a threat or a gun guy like me, in which case I'm looking at his OC gear to see if his taste is similar to mine. He has my attention because he is carrying a gun, I can't help it. I look at people.
Wouldn't the person openly displaying their gun actually be LESS likely to be a person who would have something to hide, either in the present or in their past?
Yep, but like I said...I'm a people watcher. I'm not looking with suspicion or doubt, just looking. Unless the guy beside him looks like a complete threat, I'll observe the person with the gun. I think it is almost subconscious.
What was that old military motto? "Be polite but have a plan to kill everyone around you?" I just pay attention to those in my vicinity. Just because I give someone the once over, doesn't mean I do it negatively. I give many people the once over. If they look back or notice, I nod or say hello so as not to appear rude. OC is common here, but not overly common. I may see one or two people a day OC'ing, so yeah...I look. He or she may be a threat or may be next shooting buddy after a conversation.