if it’s community specific, you can report content to the mods
if it’s instance specific, like some bad user or community showing up in the feed, or wanting to replace a moderator, you can ask an instance admin
How do you know an instance or admin is trustworthy?
There’s no consistent answer to this one yet. There’s no centralized place to find that info. Some instances are run by non profits and organizations, which is a good sign. If they are open and transparent about their financials (ex. OpenCollective) that’s a good sign. If the admin is YOU (ex. Some people self host an instance just for themselves), then you can be 100% confident since it’s just yourself.
For myself, I picked lemmy.ca because I’m in Canada and the instance seems pretty open. Last I heard they were filing for their official nonprofit status in Canada so I’m pretty happy with my choice.