You can look up the defederated instances on each single instance by simply attaching /instance to the URL. In lemmy 0.18.2 and earlier the blocked ones are on the bottom of the page, ever since 0.18.3 and up there's a separate page.
e.g. https://lemmy.dbzer0.com/instances (lemmygrad is indeed on the blocklist, no idea if that's mutual or a solitary choice by dbzer0, I didn't check the details on lemmygrad's instance page).