I would say my ability to tell if you carry isnt superb but probably better than most.
Here I will let you in on a few while leaving out the obvious (aka black fanny pack, heavy unseasonal jacket wear, photographers vest, pager-pal pager, large "walking gant" while using thunderwear, weighted-to-one-side jacket, bulging side lump on same side of body as dominant hand, over layers of untucked clothing (and even then there might still be a bulge on their dominant hip/mid-back/under non-dominant arm when they step forward or turn), or even the simple test some can do by dropping something of value and watching them pick it up with one hand while securing the firearm with the other. Anyway here we go!
#1 If they hurry across the street they will 99% put their hand on the gun to control the swing/weight/possibility of it falling whatever, their hand will go on it.
#2 Standing still for any period of time they will touch it. I am not sure if this is just that its human nature to want to know its there or if maybe they are checking to see if its adjusted right? I dont know I pocket carry...
#3 When they are close to others their hand/arm/elbow will cover/secure the side they are carrying (this can happen even if they are not carrying if they carry alot just by habbit).
#4 If someone is carrying they will likely manuever canted. Somewhat the same as above but if you see someone moving between to shopping carts or around closed areas and they are slightly skewed to one side its a flag.... and thats the side they will usually have the firearm.
#5 If someone is carrying and they do make an "adjustment" at anytime they will often times look around to see if anyone just watched what they did (unlike someone adjusting their cell phone carrier in which they dont care if you saw). Its just habbit and persona of human nature and characteristics of actions. Simply normal to be aware of your surrounds... its just another flag.
I would really like to tell you guys more but heck if anyone reading this altered their behavior due to training techniques it would make it harder for people like me who are super vigilant to tell. Hope you enjoyed!