How did natzis know who was Jewish and who was not?

Answer 1

The Nazis knew who the Jews were by using carefully structured intelligence networks located among the various European countries prior to the commencement of WW II -- a network that also extended into North/South America, Australia, and New Zealand. The information received was catalogued in Germany at intelligence centres. Virtually every family was known (and not just the Jews), their names/ages/occupations etc. Thus when the decision to liquidate the European Jews was made in 1941, rounding them up wasn't so difficult. It was made easier also by the insistence that Jews wear the star of David when in public. That made it easy to spot them when the "rounding up" occurred in specific cities. Thus if they couldn't be found at their residences/places of business, they would be identified on the street.