This material is taught to security operatives and to law enforcement in the spirit of detecting "illegal" gun carry.  I offer it here in the spirit of the fact that you should know who around you is also armed because their motivations may not be in favor of your interests.  As well in the spirit of knowing what others may be looking for as well.  If you know target indicators, you can avoid producing them.

I am not even going to get into show-giving-away moves like wearing "gun clothing".

Common Behaviors 

Security Check: Often those who are uncomfortable carrying weapons will touch and/or adjust the weapons on their bodies as the go through the day. This may be as subtle as a soft bump with the elbow, wrist or hand. Or it could be a distinct grasping of the weapon as they adjust it. The more comfortable the gun carrier is in their position of legitimacy, the less they might care about giving away the fact they are armed.  Some carrying in a less legitimate manner might wish to security check more often, but they will be self-conscious about doing so.  The manner in which they execute this may give some background on who they are and why they are present.

Unnatural Walk: Specifically with the concealed carry of long guns.  These guys may walk in an unusual manner, as if limping, and fail to fully bend their knees.  One common way to conceal a rifle by urban gangs is down the leg of their pants, or under a large overcoat.  As well, it was a common carry manner to put a small handgun in a shoe, which would lead to a limp of sorts.  Look at possible feigned disabilities, as well as out of place clothing.  Again, the total circumstances will indicate the likelihood of a weapon being present.

A man with a disability may also not bend the leg or walk with an unnatural gait, but he will probably not be nervous, nor dressed in a manner that is out of place.  A man in a business suit limping is not the same as a man wearing oversized gangland clothing, sweating profusely and muttering to himself. 

Clothing Tells: When you place a handgun in a jacket pocket, the coat typically hangs lower on the side where the weapon is located. Same thing for pants.  In addition, you will often see the fabric pulled tight from the weight of the gun, and the weapon may swing during walking.

Look for the clothing being out of place for the environment and weather.  A man wearing a knee high rain coat in Scottsdale in July is out of place.  A man wearing gangland attire when everyone else is dressed differently is out of place.  Don't dismiss this, it bears further watching.

Printing: When trying to conceal a shotgun or rifle under a coat while walking, the butt of the weapon will bulge behind the armpit. Additionally, the jacket does not move naturally because it is supported by the outline of the weapon. Much the same may happen when wearing a shoulder holster.  Also, look for the print of straps or slings under the clothing.

Visible Weapon Part: Look at the images of active shooter events and you will realize that parts of the weapons were often visible as the bad guy walked into the scene. The problem is that people do not look.  Same thing with knives...the clip on the folding knife is a tell.

Additional Considerations

Most Common Carry For A Primary Weapon is tucked into the right front waistband between the navel and hip.  Amazing that with all of the marketing done by the Modern Technique people, holsterless appendix is what the "untrained" gravitate to.  This is "bad guy carry", and the easiest to hide and hardest to detect.

Second most common place is at the small of the back, or just to the strong side.  What do all have in common?  They are at the waistband area so any cursory visual examination of a potential adversary should begin at the waistband (after seeing his hands of course).

Another good thing to do is to determine a person’s primary side.  Wrist watches are usually worn in the weak hand, except in Europe where they may be worn on the primary side.   First steps are generally taken with the weak side leg.  Use of the primary hand for everyday activities like, moving objects, writing, dialing a phone.  If one guy high fives the others with his left hand while his right hand is in his jacket pocket, it could be a tell.

For our students in areas where weapon carry is prevalent, I suggest a field study.  Go to a public place, such as a food court at a mall, a Starbucks, or any place where there is a constant flow of people.  Buy yourself a coffee and sit and watch.  Look for the tells described and see if you can spot the hidden gun.  Some will be obvious, others will be difficult to spot, and some may be imperceptible. 

Consider that it is quite possible to hide the weapon so deep in the clothing that anything short of a pat down will not reveal it...but a speedy draw will not be the result.

There is a move in some circles as well to pioneer thermal detection devices to "spot the gunman".  I suspect these can be easily thwarted with a bit of cleverness, and that may be the basis for additional study.