Take note that most adults are a little inhibited about staring at other people and will not be checking you out, wondering if that bulge, where ever it is, is a weapon. Others are not so reserved, especially children, and just might be better at spotting something a little odd. You might call that the "emporer's new clothes" syndrome. On the other hand, as another just metnioned, if you are more firearms conscious, you tend to notice certain things, like people who seem to be overdressed on a hot day or acting a little weird. Other things about them seem go fit the image, which I imagine is something you want to avoid.
For instance, once I was at the gas station and another man was standing there filling his gas tank. It was a hot day and he was wearing a vest, he was continually looking around, and there was a bunch of those nylon cable ties on the rear view mirror of his truck. Now maybe he was a policeman, only he looked much too old for that but you never know. I have no idea whether or not he was armed or not but he caught my attention.