Does it fit the person who's selling it? And if not, do they volunteer that that's why they're selling it, without being asked?
Don't ask them over the phone if it has any dings, ask when you get there, and see if what they tell you corresponds to the bike's actual condition. Same with the condition of consumables like tires, brake pads, cable housings, chain - ask them when you get there how many miles or months they have on them, then do your own inspection. If it's really their bike, they'll know that stuff.
If they're a career thief, they still may know, but I would think you could eliminate a lot of casual thieves just by finding out whether or not they actually know what they're selling.