Like with anything, pictures, pictures pictures. Then it is easier to assess parts and condition, thereby allowing for a better estimation of value.
Unless its been hacked, chopped or bubbad beyond repair, or is worn out to the point it rattles like a jar of bolts, you will be able to get at least $400 for it.
If you are buying, you may get laughed at if you offer $200, but otherwise $400-$500 is a fairly common starting point.
In regards to the Bavarian Police carbines, the Rural ones tend to be the most common ones encountered. If its original from the Bavarian police, the Carbine should have the last four digits of the serial number stamped on the op rod slide, in the slingwell of the stock, on the handguard, and on the trigger housing. There may be some other spots with some other stamps, depending on the specific county in Bavaria.
Sometimes the Germans got their M1's fairly early in the whole rebuilding process and you still see some once in a while with original early parts, flip sights, no bayo lugs etc etc. They make for a very interesting Carbine collecting niche.