I think the admin could share that information in the topic about the category, something like: “This is a category with limited access only for trusted members to discuss more personal topics.” This is what I usually do.
As a user, you can also see groups with access to the category in the share modal:
Though of course, you never know if more groups that aren’t visible to you have access too.
But even knowing who has access to a category now doesn’t really tell you who will have tomorrow. The topic could be moved to another category, or the permissions for the category could be changed.
So you need to trust the forum’s staff that they keep it private, so I think you can also trust them to tell you about who has access.