I don't carry stuff but I've done so a bit and otherwise experimented with concealment a whole lot. One of the main things I'd find myself doing is worrying about concealment. I.e., is the weapon bulging thrugh clothing? has the shirt or jacket slid up over it? Those sorts of things, which would make me periodically look and feel, often correcting my clothes. Pulling down the shirt or jacket that is. If people do that over and over it's either that or they must be very self conscious about their boxer shorts or something.
Some times, if the weapon was large or simply badly held in place, such as i.e, sticking a ballpeen hammer in my waistband, it'd make me walk funny, almost as if I had some pain or something, all as an effort to stabilize the weapon so it didn't fall down my pant leg or have the handle start bulging during normal walking motion. Those might be things to look for too. Usually this will mean, slower pace of walking, shorter steps, and maybe an awkward way of doing it.
It's downright scary what can be concealed with a little bit of effort. Biggest thing I ever managed to successfully conceal was one of these https://www.youtube.com/watch?v=u15y2-gwKc8 I simply stuck the handle my waistband and tucked my jacket over the bladed part, which rested flat against the abdomen. The only revealing thing about it was the handle going down from the waistband, which would bulge unless I walked in a certain way, but it was still disturbingly well concealed. I was wearing completely joe normal clothing, so if it had been baggy pants and so on it could have worked even better. The next biggest thing was a knife similar to this one:
That's a lot of knife... I simply made a cardboard sheath, reinforced the top with some pieces of plastic I broke off from something and taped it up. No fancy stuff, any street punk could do it. And I attached it with safety pins to the inside of a denim jacket I used to have. Probably wouldn't work on a synthetic fibre jacket though, as I expect it would hang too much with the weight on that side. It hung vertically straight down, with the handle on top, very easy to access so long as the jacket was not closed. Also, having the jacket open actually made it easier to conceal as it would bulge less. You probably won't see much of that kind of stuff though. But I expect there to be a few people carrying on this level too. I mean, you see it on the news from time to time. Someone gets stabbed and/or murdered in the middle of town or some such place, with a big knife and even small swords. I remember some gangster type kid being arrested over carrying something like that in this town maybe last summer or so. Because he pulled it out and threatened someone with it... But do people ever take notice before that point? ...they're out there too. The freakish ninja carriers.
Normal carry... Perhaps more relevant. Folders, I'd usually clip them on the right side inside either the waistband or the pocket, depending on how much the shirt or jacket conceals. Or of extra covert I'd simply slip it in the pocket alongside the cellphone and so on. When sitting I'd be periodically checking that it wasn't falling out of the pocket, maybe pushing it to the bottom of the pocket. Can't remember ever having done that with cellphones and other crap. Seems like a weapon (or "stuff-you-want-to-keep-hidden") thing.
Fixed blades I'd usually carry in the waistband, in as horizontal a position as possible. Easy access, easy concealment. Having them on the sides or back just seemed to not work as well for keeping them covert so I'd carry them right behind the belt buckle area. Most of the mannerisms were centered on pulling the shirt down and checking that the unfastened end of the sheath wasn't sliding up out of cover, pushing it down a bit if need be. (I'd attach the top end to a belt loop or something.)
...I don't know how to summarize that. Err... The waistband and general hip and lower abdomen area all the way around the body is what you can refer to as the "weapon belt." Most weapons will be there. Hands doing stuff in that area in general, I would label suspect. Especially if it isn't just picking something (cell, chewing gum, etc) out of a pocket or simple body language, like hands on hips. I don't think most people do things like weapon-check motions and pulling at their clothes normally, at least not with any noticeable frequency or pattern. I guess hands may be something to watch. What do people do with their hands? Where do they go? And how do presumably non-carrying peoples hands go? You want to watch hands anyway when you talk to people in case they're palming a knife or something so it might be a useful extension of that habit to simply watch where the hands go when they think nobody is looking.
That's something on the basic awareness stuff anyway. I don't think it can be skipped. But the psychological angle that Russel Sage mentions seems like fertile ground for exploration. It's the stuff that can give you that seemingly psychic jedi ability to have a sense if someone is carrying just by looking at how they behave psychologically so to speak. The hardass 90 pounder and so on.