It's going to depend on the instance and what software they're running to connect to the fediverse.

For PieFed instances, visit the homepage for the instance itself, scroll all the way to the bottom and click on the "About" page. There's usually some contact info for the admins there.

For Lemmy instances, visit their homepage, scroll to the bottom and click on "Legal". There's usually contact info for the admins there.

For Mbin/Kbin instances, click on the menu, then scroll down to the "contact" link.

Sometimes instances won't let you view information without a local account. Usually, in those cases, you can try to "register for an account" but then look for any sort of "privacy" or "terms of service" links which should have contact information for the instance admins.