> I need to read more sci-fi

Or just be a little thoughtful. If I want to persecute homosexuals, I can go ta a gay bar and collect DNA. If I want to persecute crypto-Jews, whenever we bust a place where some of them have gathered to worship, we can catch the others in the area by collecting DNA.

In the general case: we can designate a place to be an enemy place, and arrest the people who we can prove have been in that place.