While hitchhiking you can get a lot of information from the number plates of cars. On many cars you can find stickers or even parts of number plate that point out to the garage (and town) where the car is bought. If you're looking for long hitches, this could come in handy as it gives you important information of the hometown of the driver.
The lists of number plates are from german Wikipedia, just because there is mostly more information (like maps) than in the english version.
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.
On russian number plates there is always a number over the russian flag on the right side of the number plate. This indicates where the car is from. For example 72 stands for Tyumen.
On  you can find a list of all russian number plates.
Same as Germany. On  you can find a list of austrian number plates.
List here: 
Polish number plates start with 3 Letters. The first one tells you the region, the second and third one specify the region.
List:  (the list on the right should be enough)
First two letters indicates the origin of a car.
On the right side on the bottom is a number. The number tells where the car is from.