This question (and this one written in parallel) comes out of discussion on a more specific question with details about a particular situation, which was apparently too specific for this site.
When plainclothes officers driving an unmarked car want to arrest a person, how do they identify themselves in a way that isn't easily faked, especially when they're acting against somebody who has government-mandated duties to protect items in his custody?
This is different from "(When) are police required to identify themselves?" because that question is about getting the name and badge number of a uniformed on-duty officer, where it's clear they are police but not known exactly who/which specific officer they are. This question pertains to individuals who are not wearing a uniform and not driving a marked police car, and who are making demands of another person, including detaining that person and taking them away against the person's will and in violation of the person's duties.
While the option of calling for backup by uniformed officers in a marked car could be one strategy, suppose that the people who claim to be police are not using that option for whatever reason (maybe because they aren't police, maybe they just don't want to).
This is also related to "How can you tell if you have to follow a police officer's instructions?" but for the case of someone who is not in uniform and just claims to be a police officer.
There are examples of people who kidnap others by claiming to be police, and it seems to be illegal to resist such a kidnapping (see motivating question about that specific situation here). Not all the people who use this strategy have made the news and even of those who have, not all have been caught. It's common enough there are even pop-culture memes about people falsely claiming to be the police not having to identify themselves ("We don't need no stinkin' badges!") but the meme may or may not match the law.