A Guide to Looking Up License Tag Numbers. If you have ever been asked to lookup someone on the Web employing simply a license tag number, you in all probability understand by now that this is no easy project. Since there is great demand for a free on-line license plate search application, no such service actually exists. If you need to ascertain the owner of a truck using a license tag number, you will want to either get the assistance of a law enforcement officer, engage a private police detective or get the assistance of an online investigation service which offers up license tag search services. Beyond these options, you don’t have any other way to explore license plate numbers on your own.