How did the Nazis know if you were Jewish?
2. In Germany and Austria anyone 'suspected' of being Jewish was asked to produce evidence that they were not Jewish, for example, by producing certificates of baptism for all four grandparents.
3. In many other countries the Nazis relied on informers and in Poland and most East European countries the majority of Jews lived in obviously Jewish communities.
There was a bitter joke that circulated in Germany at the time: 'What kind of women do German men like best?' Answer: 'No, not blondes. Aryan grandmothers!'
The above doesn't really address this very important question all that well. Some of the things I would point out are below...please add to them - there are many more.
The identification, or singling out, the entire process was gradual, not done in just one swoop. There were a number of progressive steps over years. There wasn't just an announcement "all Jews report to the station and prepare for extermination".
The prevalent steroetypes were well known and exploited.
The Jews in Eastern Europe and the Balkans at the time were for the most part not as assimilated as those you see in the US and many other places now. They were generally much more of the Orthodox type you can still sometimes see, which are easily identified by their dress and style. Also, they weren't hiding...in fact they were proud of being Jewish and would say so if asked. They were generally members of an identifiable community and attended a Temple, had certain days of worship, dress and traditions (like eating only kosher foods) that made them easy to identify. Then as now, most people are not going to deny their religion easily.
However, at first it was just stirring up hatred toward the Jews by the society that helped identify and isolate them. Then, step by step, making things they would do illegal or something you had to register to do, requiring them to live certain places, identify themselves (rememebr they were law abiding people), not allowing them to work (and as others wanted the jobs, others would ID them), and more that hepled the government know who they were. Many non-jews, as anti-semitisim was considered proper and even promoted, would make sure anyone they knew to be a Jew was identified. Even being a Jewish "sympathiser" (growing to be someone who didn't expose hate toward them) wasn't accepted - laws were passed requiring neighbors to report anyone they knew was a Jew or sympathiser and not doing so itself was punishable. (Obviously, the penalties could be horrific). Rewards, frequently that the one turning a Jew in would get to keep much of the Jews property were common. Certainly some people were wrongly identified as Jews, because of something like having a big nose or having ever been seen going to a Jewish area or business. Many were "set up" by others who had a score to settle. Being mean, even killing Jews was OK - and unfortunatley then, as now, there were many people who just wanted to be mean.
Identifying one, identified the family - not just by custom, but because the family wouldn't abandon one of its own.
Certainly many mis-identifications were made, based on predjuces and stereotypes, like having a large nose, or such.
They were required at one time to move into and only live in certain areas - or ghettos. Anyone in there was able to be presumed to be a Jew. Not allowed to work, etc.
Remember, they were, and were (perhaps) fearful but also proud of being Jewish.
Neighbors turned them in...and were rewarded for doing so, being frequently given much of the family's belongings. (Yes, again there were many instances of someone lying to get revenge on another).
That even progressed to NOT reporting one you suspected as being punishable as treason...and you and your family could be taken away or killed as a "jew sympathizer".
Jewish men, then and now, were circumsized. The Nazi official could require anyone, anywhere, to drop their pants and prove they weren't Jewish.
Many Jews followed certain customs, laws and rituals that would expose them or could be used to do so- if they keep Kosher for example - forcing them to eat pork was vile and offensive, even phyiscally impossible for someone brought up to avoid it. Wearing a yamulkhe or prayer shawl was a dead giveaway.
Having any "Jewish things" in your house, prayer books, picture of "obviously Jewish" relatives, all were enough proof. (And consider Jews are required to have a mezzuah with a certain prayer on the doorposts of their house).
1. Germany and (from March 1938) Austria
From April 1933 onwards Jews were banned from various kinds of work and activities, starting with the dismissal of Jews employed in the public sector. Then, later that month a large number of Jews were expelled from the German universities on 'racial' grounds. Then Jews were forbidden to run theatres or act ... So, how did the authorities actually pick the individuals?
Already before the Nazis came to power people in many parts of Germany were intensely Jew-conscious. 'Is he/she one [a Jew]?' was considered very interesting and very spicy. (The situation was very different from that in modern Britain, for example).
When it came to picking people out, co-workers and bosses in that Jew-conscious society generally had a pretty good idea of who was a Jew or of Jewish origin. In the cases referred to, the authorities dismissed the people they thought were Jews. The victims of these acts of discrimination then had the option of proving they weren't Jews. This involved producing the notorious Ariernachweis ('Aryan certificate') based on certificates of baptism for the parents and grandparents. On the whole, the authorities included people they though might be Jewish, and then let them produce evidence to the contrary.
In Germany, the Nazis were particularly bothered about 'secret Jews'. Nazi propganda worked with conspiracy theories that claimed that there were ethnic Jews lurking, so to speak, in all kinds of unlikely places, with fingers on just about every imaginable lever of power. So the tendency was to require more people than necessary to produce those certificates. The work involved was at times almost crippling for the Protestant and Roman Catholic Churches in Germany, but they collaborated in this shameful exercise.
Here too there was an obsession was with 'secret Jews', with atheists, Communists, with perhaps one or two Jewish grandparent.
As mentioned above, roundups took place in stages, often by category (for example, stateless Jews first).
In France the government had already done some of the rounding up as many refugees from Nazi Germany were interned in camps ...
Nevertheless, in France and Belgium the Nazis were not on home ground and the proportion of people who managed to escape deportation to the death camps was higher than in Germany, for example.
(In most occupied territories, research on grandparents was usually not practical or was considered too cumbersome and time-consuming; and there were also linguistic problems).
There were degrees of being Jewish (half Jew for, obviously, one parent Jewish the other not) and there was also the problem of some one converting to another faith, perhaps generations ago, which the Church might defend. All of this made for some grey areas for the Nazis in deciding who was to be deported. There were sometimes well known figures protected (and sometimes not , Harry Gold a famous Polish composer died in Treblinka, Sigmund Freud's sisters, and so on).
The Nazis also used census returns and there were of course records kept of church and synagogue members, marriages, military and everything else just like today.
The "Jewish councils" (Judenrats) also helped prepare lists for those to be deported to the extermination camps --a certain number, say 5,000, was demanded on a given day and they hoped they could placate Nazi demands or "save some" by working for the German war cause, for instance. None of this helped in the end, since they were dealing with one of the most bloodthirsty group of fanatics ever.
See the related question.