Determining whether guests can view a given forum


I am working on some logic in the to determine whether a given forum is accessible to guests/mods/admins etc and behave accordingly.

To be specific, I'm tying in a 3rd party service on the Post DataWriter that deals with new posts on a forum differently depending on whether the forum is accessible to guests, mods, admins or whoever.

All the permissions functions I can see are related to the current user, or a given user, so maybe I need to prep a guest user's perms and supply them to a relevant function?