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.
Number plates in Germany look like this: B - AA 1234 . Here the first letter (often two, sometimes three) can tell you from which municipality/city a car is coming from. For example B means Berlin, OS means Osnabrueck etc. Even a lot of drivers know the number plate letters, so hitchhiking with a sign saying OS, WB (Wittenberg) or something else will be widely understood if you're near that city. Don't try to use a letter from a city in Bavaria in the north of Germany, of course.
- On Wikipedia you can find a list with all German number plates.