I'll make this a wiki answer so others can add to it...
The first and most obvious thing is the old saying "if it looks too good to be true, it probably is".
Look at the condition of the interior, particularly the driver's seat and steering wheel. Does the pattern of wear match what you'd expect for the stated mileage?
Paperwork - do they have anything to back up their claims? In the UK, the mileage is recorded at each year's MOT (roadworthyness) test, so that's usually a good reference. Again, if they've 'lost' the paperwork, be suspicious (in recent years they have computerised the MOT system, so you can look up the last few tests for a car)
Personally, I'd avoid a very low milage car just as I'd avoid a very high milage one. Cars don't generally like sitting around for long periods unused - rubber parts perish, things sieze and stick, etc. Go for a decent, honest, average-miles car that looks like it has been looked after.