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