The most entertaining case of a guy I saw carrying...
... was in the late 80's. Older gentleman was carrying in plain sight at the Fashion Square Mall in Orlando. NAA mini-revolver in a belt buckle holder. Those were still fairly uncommon, then, and virtually everybody seemed to think it was just a decorative buckle.
As far as telling if somebody is carrying, there are various cues you can pick up on, that others have already covered. My rules of thumb, though:
1) Assume everybody IS carrying. Better to be surprised when they are not, than when they are.
2) If somebody's behavior makes you nervous, don't worry about whether he is armed. Your subconscious has probably picked up on cues that you haven't consciously processed yet. If your internal alarms are going off, stay away from that person, pay even closer attention to your surroundings in case you need an escape route or a police officer, and prepare to vacate the area.
3) If you can consciously determine a valid reason why that person is making you nervous, consider calling the police.