The minute you enter a township in Germany there is a rectangular yellow sign depicting the name of the place you are in. Once inside that area the speed limit is 50 kmh - it does not require specific posting. When you leave the same thing is posted crossed out diagonally in red.

Assume you were caught on a rural road and not an Autobahn?

If you went into a traffic-quiet zone (like a school area) there would be 30kmh signposts (black letters on round red-ringed posts).

So if your notification is accusing you of exceeding the 50kph limit in a built-up area then tough luck.

Happy Driving in The Fatherland