Look at posts and comments from the instance admin. Does this person seem to know anything about running a server or managing software? Do you like the admin’s tone and attitude? Does it seem like the instance will be around for the long-term, or is it someone’s hobby that they may abandon on a whim?
Look at the instance’s ban list. Do you agree with the choices? Maybe you want your All feed to include exploding-heads and lemmygrad, or maybe you don’t.
Does the instance seem to have a community theme that you want to be associated with? Some emphasize LGBTQ+ communities, or NSFW content, or particular political views.