This is a question that comes up fairly frequently on this subreddit -- you might be interested in some previous threads that touch on this topic.

• "How did Nazis identify who was Jewish or not?" (roundup of past answers by a deleted user)

• "Did the Nazis treat Christian Jews differently?" answered by /u/kieslowskifan

• "How were Jews identified by Nazis? What happens if a Jew didn't wear the Star of David while walking on the streets in a Nazi-occupied town/city?" answered by /u/ArjanB, /u/Jan_van_Bergen, /u/commiespaceinvader

• "Out of curiosity, what would have happened to my family during the Nazi rule?" answered by /u/QuickSpore

• "If Jews converted to Christianity in Nazi Germany then were they still persecuted? How exactly did the government determine who was Jewish or not?" by /u/Alexs000

• "How did the Nazi's know who was Jewish?" answered by /u/shortandfighting