I agree that stashes should be short-lived, but

  % git stash list

will show you what branch was checked out when you stashed. Also, because a stash is represented as a commit, you can know what commit the stash is based on. From the git-stash man page:

"A stash is represented as a commit whose tree records the state of the working directory, and its first parent is the commit at HEAD when the stash was created. The tree of the second parent records the state of the index when the stash is made, and it is made a child of the HEAD commit. The ancestry graph looks like this:

         .----W
        /    /
  -----H----I

where H is the HEAD commit, I is a commit that records the state of the index, and W is a commit that records the state of the working tree."