zlacker

Places like CarFax collect data from maintenance done on the vehicle. In which case you may be able to detect mileage rollback because the number goes negative between two visits.

But a private vehicle fleet may use private mechanics, who probably do not report information like that (there's no financial incentive to do so, and there's probably no mechanism to make it easy).

I might be able to tell that this Dodge Dart has had the frame straightened twice and the engine doesn't match the VIN number. But a private fleet vehicle is probably a black box.