Agreed on the condition in relation to the stated miles, it's pretty much the only way you can tell. You really have to look the car over closely if he is representing it as a 50,000 mile car. If the miles are really that low then the car should be in good shape with very little wear. Obviously a 150,000 mile car will look much more worn than one with 50,000. The dead giveaway for true miles is the interior. Check the carpet near the pedals for frayed carpet from the heel, look at the steering wheel and driver seat for excessive wear. A 50,000 mile car will have little wear in these areas. Be cautious if some parts of the car that typically wear out look newer than the rest of the car, like the brake pedal pad, the carpet, the windshield or if the front nose has been repainted. It might mean he replaced worn parts in an effort to make the car look younger than it is and you need to start asking him tough questions about the car's past.