If it was one of my very few family or friends that know I carry, and they had asked discreetly. I would answer their question with a nod or shake of my head, assuming that they would have a legitimate reason for asking.
I have been asked in the past by casual acquaintances who know I shoot. My standard response is to mime drawing a six shooter from where my cowboy holsters would be, then look at my empty hand and say "um, apparently not." and then change the subject.
When asked by other acquaintances who are not well known to me, if I think they are anti's I simply reply with "carrying what?" and then I politely disengage with them. If they seem pro-gun I will mention to them what a personal question that is and they might as well be asking me what kind of underwear I wear, or my wife what is in her purse. I will point out that "carry" is usually accompanied by another word - "concealed". And if you go around talking about it you are not being very concealed. If I am not sure about their stance, I assume they are antis and react accordingly.
Now a few years ago I was on an outdoor field trip with a bunch of second grade kids. I was bringing up the back of the group when they all stopped on the trail. I worked my way to the front of the group where the principal and teacher from school were keeping the kids back away from a rattlesnake that was resting on the trail about 20 feet away. The principal, who knows I shoot, looked at the snake, then me, then my waist where she assumed that a concealed gun would most likely be carried. Then she said "Do something about it." So I did.
I scrounged a couple long sticks, pinned the snake to the ground then immobilized its head. Picked up the snake, and allowed the kids to have a look at it from about 6 feet away. Showed them its fangs, dripped a little venom out of them. Then lectured them about types of poisonous and non-poisonous snakes, and not getting near them, etc. Then after they all moved on up the trail, I moved a ways off the trail, killed the snake, buried the head and rushed to catch back up with the group.
When I got back to the group the principal walked up next to me and said "That was good. But it wasn't exactly what I had in mind." I smiled and said "I know."
Six years later she and I were talking one afternoon and she said "I have known you for 12 years. I have been out shooting with you. And I still don't know. Where you carrying that day?" I smiled, nodded and made her pay for lunch.