Absolutely. E2EE is maybe the most important sinlge feature in digital communication.

That’s not specifically an issue for the Fediverse but your entire online life. You may consider a service like Keyoxide.

Edit for an addition: I just noticed that this doesn’t answer the question. Keyoxide helps to protect against impersonation, not how to find out whether that already happens. I don’t know of an easy way to do that, though. You’d probably need to do a lot of research across the web I guess?