While hitchhiking you can get a lot of information from the number plates of cars.
French number plates end with the number of the département the car is registered in. For example, Parisian cars end with the number 75, 78, 91,92,93,94,95. See List of arrondissements of France and French vehicle registration plates at Wikipedia. New number plates are to appear in the nearest future and those will not carry any information about the region, unless the owner decided to put a department reference on it. The current plates already in use will still remain for a while.