Editor's Note: This tip was updated in July 2015. The original version was restricted to xrefs only, the author notes, but ordinary blocks can also be XClipped, so the updated file includes them. It still has the part that limits to xrefs, commented out, with instructions to restore it if that's what the user wants.
Kent Cooper, as he frequently does, responds to a request in the Autodesk forums with a custom-made routine that he shares with us. This one, named XClipID.lsp, finds any xrefs in the current space that have been clipped with the XClip command, and draws a heavy red polyline around the clipping boundary of each for easy visual identification. After you load the LISP file, run the command XClipID.