system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In WWII, How did the Germans know who was Jewish or not? Couldn't they have just covered it up?

I love WWII stuff, but this is never really explained other than people not having the proper ID or they looked stereotypically Jewish.
gpt
They used three main approaches. First, everyone had to register themselves and their ancestry, and to get the most privileges, you needed to prove your status as a pure Aryan by providing your pedigree. So if you couldn't prove your racial status, you were already disadvantaged, even if evidence did not necessarily suggest you were a Jew.

Second, in the beginning, many Jews practiced their religion openly despite discrimination. Even non-practicing Jews often had clearly identifiable Jewish relatives or sufficiently participated in the Jewish community to cast suspicion on them, and friends and neighbors were often willing to identify Jews they knew, under coercion or with the promise of a reward.

Third, even if you were safe from being identified socially, the Nazis made a point out of going through civil and religious archives to identify Jews (including people with distant Jewish relatives). You could try to pretend ignorance about some of your relatives and accept you could not prove pure status, but that does not mean you would not eventually be identified as a Jew from other sources. Consider that the Nazis were only in power for a little over a decade--before that, Jews had freely identified themselves on taxes, census records, etc. and this data could now come back to haunt them and their relatives.

Lastly, it's worth noting that for all the strict rules involved, the implementation was not so rigorous in practice. Local government officials had great power, and with the right connections some Jewish ancestry might be overlooked--conversely, it was no big problem if an official erred on the side of caution and officially proclaimed someone a Jew even if they were not.