javascript:(function(){f='http://twitter.com/share?url='+encodeURIComponent(window.location.href)+'&text='+encodeURIComponent(document.title);a=function(){if(!window.open(f,'_blank','width=680,height=249,status=0,scrollbar=0,location=1,menubar=0,left=839,top=116'))location.href=f};a()})()

A bookmarklet to report the rev=canonical attribute if one exists, or alert if one does not.

Full copyable text:
javascript:(function(){var url=document.location;var links=document.getElementsByTagName('link');var found=0;for(var i=0,l;l=links[i];i++){if(l.getAttribute('rev')=='canonical'||(/alternate short/).exec(l.getAttribute('rel'))){found=l.getAttribute('href');break;}}if(!found){for(var i=0;l=document.links[i];i++){if(l.getAttribute('rev')=='canonical'){found=l.getAttribute('href');break;}}}if(found){prompt('URL:',found);}else{alert('No canonical url found.');}})();