I've been a people watcher for as long as I can remember but intensified it lately to help me spot BG's from further off. In the process I've spotted more than one CC (one an off duty LEO) and a couple of folks I knew by name but not by sight.  So far, no BG's (TG).  The techniques I use are some that were discussed here and in another forum and are based on what police watch for to try to ID CC.  It's surprisingly easy -- easier than I would have thought.  Do a Forum Search and I'm sure you can find other threads.

Dave...good article.
If you are going to carry concealed....on your waist / hip...........you need a good holster and a good belt. "New" people like to touch / check their gun, "just in case" on a regualr basis and are easy to spot.  If you are going to ware a shoulder rig, your fleece, jacket or shirt need to be at least the next larger size. 
Also, people that carry on a regualr basis tend to reach for / to their gun if something like a car back fire startles them. Watch and see this for yourself when out and about...somebody drops something, or a car back fires or there is some other loud bang....cops in plain clothes are "bad" for this, they start to reach for their gun while scanniing to see what has occured....dead give away.
Try out your holster and various clothes with your spouse or a friend looking at you with a critical eye, looking for the bump, or sag of your belt or pocket.
The quicker or easier access you have to draw your gun, the more likely it is that someone else will see it. The more layers of clothes it is under, the longer it will take to draw and fire. Practice drawing from how you carry.
When still an LEO, on duty; if I spotted a stranger carrying concealed; If I could with out creating a scene, I would usually pick a spot, out of the way, to ID them and request to see their permit...low key approach. If "off duty"...I would keep an eye on them for a while and see if they appeared to be "up to anything". If I knew the person I would mention to them that they were not "concealed".

I spot CC people all the time.

Spotted a guy a while back at a local restaurant who had a pager pal. He adjusted his pistol every time he sat down after going to the food bars for more groceries. Pretty obvious. My friends and I were the only ones who seemed to notice though.

attached is a second article on the topic

HTack,
Actually it is a word document at the bottom of the post.  If you click on it, it should open on your computer.  If not, send me a personal email and I will forward to you.  Dave.Leonti@verizon.com

I'm with T Berry. I spotted a guy recently at a McDonald's. He was leaving as I was coming in. He had a baseball cap on from one of the "schools" and a sleeveless fleece vest (that should have been a size or two bigger) and a short sleeved shirt. I saw the hat and immediately checked out the bulge on the right hip.
I try to stay away from anything 5.11 or other "tactical". I like John Farnam's idea of being "vanilla" and blending in. Some of my friends carry their folders in an "appendix" carry. so that it is not readily visible (as opposed to right/front pocket).
IMHO, the big thing is to plan your wardrobe around carrying, carry all of the time (when you can) so that you are used to it and get a good quality comfortable holster. Personally, I like my Crossbreed Supertuck.