I have heard about the NAZI's using Circumcision as a method of determining whether a person was Jewish or not. This was a ridiculous system, and would have meant that someone like me would have been wrongly classified as Jewish (or Muslim) or indeed, American - where most Boys are Circumcised.
A bit of history here. While (Nazi) racial theories did often mention circumcision as being a Jewish custom, mostly as a means of ridicule and somewhat easy off-the-cuff identification of Jews, the Nuremberg Laws assigned a person's race solely according to ancestry. Physical characteristics did not play a part in determining race in Nazi Germany. Being Jewish or not depended on one's parents and grandparents being or not being Jewish. When it came to determining who was legally Jewish, only ancestry counted. This was then proven via birth records, baptismal certificates, marriage certificates, entries into parish books etc.
And also, it was up to citizens to prove they were of German/Aryan stock. This was proven by having an Ariernachweis (https://en.wikipedia.org/wiki/Aryan_certificate)
In the East things were undoubtedly much looser and German authorities did not work so meticulously to determine a person's race. Let's say they erred on the side of over-zealous. And perhaps in individual instances circumcised individuals were identified as Jewish without much else needed.
But even racist Nazis knew that Muslim circumcised and that good fine German males might suffer from phimosis or the odd results of infections due to venereal disease. They also knew there were many non-observant Jewish Germans who did not circumcise.
Determining who was Jewish was done via official records, documentation and paperwork. Not circumcision. Luckily so for American POWs I might add.
Sorry for the history, but so much nonsense is often told about this.