Unless it's a serious print (you see a clear outline through the clothing) or actually see the pistol, then you don't "know" for sure. You just suspect.
@draco88 did a good job in describing. There are "tells" to look for.
You know all those questions you have about what type of holster to carry, what kind of gun, where and how to carry it, etc? Well, all of those concerns are actually good starting points for knowing where and what to look for.
Winter months are, of course, the most difficult times to pick up telltale signs of someone carrying, because people wear outer clothing like jackets, coats, sweaters, etc over their regular clothing. But:
- Anything that's not tucked in that comes down over the beltline MAY be an indicator of concealed carry.
- "Printing" or bulges in places where people commonly carry concealed weapons, like at the waist line or maybe under an arm (for shoulder holsters).
- Unusual clothing for the season, place, or event. Like wearing jackets in warm/hot weather, wearing long shirts in places where shirts are worn by few people (like beaches), etc.
- How a person comports themselves. This could be stance, how they interact with others, suspicious acts or postures, placing themselves so that their concealed weapon is less likely to be contacted by others, etc. People who carry, for example, when they encounter friends/family may hug such that they are the ones who place their arms under the other person's arms...this keeps that person's arms above and away from their firearm, decreasing the odds that the person will touch or otherwise notice they're armed.
- Sometimes the hands give signs...along with other things. suspicious acting individuals in suspicious circumstances and suspicious places with their hand/hands always in their pockets or under their hoodie, for example.
- People new to concealed carry tend to "touch" or "check" their weapon a lot...to be sure it's still concealed, to "hitch it up" or otherwise adjust it, and just out of nervous habit about something radically new to them.
It's a good game to play with oneself for a number of reasons, to try being observant enough to see if you can pick out people who are carrying concealed.
