I'm wondering how did Nazis know who is a Jew and who isn't, there isn't a specific document which says your heritage or religion and you could't take DNA tests to reveal where do you come from or from what people you come from. Maybe my question is stupid but I could't find the answer to this anywhere.
Hi there, there's always more to add, but this thread and this thread from /u/commiespaceinvader, are good starting points while this one from /u/gingeryid might also be of use.