Questions about a car from the past!

Some considerable time ago, our S-I-L traded in a car to a dealership. He has now been contacted by the current new owner as to whether the mileage is genuine, and if it has been in an accident.
I advised him that he is under no obligation to supply information about this car. Now that historical log books no longer exist, how easy is it for a vehicle's history to be traced to previous owner(s)?