Template:Shelf:Ancestry?
Usage
This template takes one unnamed parameter, and looks for an ancestry page for a shelf with that name. Returns non-blank only if the ancestry page exists and is apparently properly set up.
Internals
This template pings the ancestry page, and checks for the correct response by {{Shelf:Ancestry}}. The correct ping response cannot result if the ancestry page doesn't exist, and is wildly unlikely if the ancestry page isn't properly set up. This template does not use the #ifexist: magic word.