At least for your instance, or for looking one instance at a time, you can see which instances they federate and defederate with by scrolling to the bottom and clicking "instances"
Or go to the url of the form "<instance domain>/instances"
You need to be on a web browser, and at the instance you want to check. Scroll all the way to the bottom, and click instances. That example is from your home instance.
I'm still trying to understand all this. I thought all instances "see" (federate) each other. If they can defederate (block) others from seeing an instance, isn't that going to make discovery a problem? Wouldn't that also mean I need to create a new account just to see that instance?
For most instances the default stance is to federate everything, and defederate as last resort. Defederatipon isn't used just because you disagree with someone's taste in memes. It's used to isolate servers that are doing something you consider unhealthy for your community. A common example is unmoderated hate speech. It's better to defederate the problem server than to expose your own users and risk an exodus. And then if/when the instance fixes the problem that caused the defederation, you can refederate them.
Sorta, but as long as they default to federate everything you could always view it from another instance, or even start a new one yourself (inconvenient but doable).
I'm using the spoiler tag because elsewhere I've seen this warning about the site:
run by people from kiwifarms (a known bad actor on the fediverse).
I'm unfamiliar with "kiwifarms" but the site currently does has some offensive language at the bottom that's associated with 4chan, so be warned.
That said, it appears to be the only site currently available to see which instances in the fediverse have blocked/defederated your instance. (The reverse of <instance domain>/instances, which only shows which instances your instance has blocked/defederated.)
They have a habit of doxing trans people for simply existing on the Internet. Also they've been responsible for several people being swatted. They were so awful that Cloudflare finally stopped having anything to do with them. Things have to be really bad for Cloudflare to nope out, it's a short list also containing the Daily Stormer and 8chan.