ID must match title or insurance card which are cross referenced to the VIN number. If they do not have ID with them, say it is locked in the car along with the keys, then a locksmith normally open the door, grab the ID and title/insurance card and check before allowing them access to the car. If something doesn't match, the door is locked and shut and the authorities are notified.
I believe they can contact the police and verify that the license plate belongs to you. I had to show my driver's license to the locksmith (he probably already called the police after I gave him my license plate number over the phone). You would (or should) know who the registered owner of the vehicle is if you aren't the owner. I would know that if I'm driving my dad's car, then he's the owner.
They check the vin as well as upon unlocking vehicle, they check insurance card and registration against the person's license
If the keys are in the ignition and the car is locked, it is a safe bet the person who wants the car opened is the one who left the keys there.
insurance card and registration
Being that the registration is locked in the car and alot of times people drive vehicles registered to someone else i.e: mother, father, significant other
