I know that one of the things they did was make people who were Jewish stitch a Star of David onto their clothes, but how did they enforce this? How did they know who was Jewish?
There have been several previous answers about this: here and here by /u/commiespaceinvader, here by /u/gingerkid1234, and here by /u/NetworkLlama (with some other answers in the full comments that may be relevant as well). If you're particularly interested in how the early Nazis identified Jewish people within Germany, I think this one would be the most relevant.