The thru-hiker guidebooks usually let you know if there are tent sites at any particular shelter. For the Companion, to which we contribute, if it says "camping" then there are tent sites. If it doesn't say anything, then there probably aren't. Places that don't allow tenting are also marked (e.g., the Partnership Shelter). This is not a foolproof method, nor does it guarantee that you'll get a site, of course.