a_link.linkXmp.getProperty("http://ns.adobe.com/xap/1.0/", "xmp:Rating"); 

var _rating = new Array; 
var _links = app.activeDocument.links; 
for (var i = 0; i < _links.length; i++) 
{ 
   try 
   { 
      _rating.push(_links[i].name + ': ' + _links[i].linkXmp.getProperty("http://ns.adobe.com/xap/1.0/", "xmp:Rating")); 
   } 
   catch(e)  
   { 
      _rating.push(_links[i].name + ': ' + e); 
   } 
} 
alert( 'Rating:\n' + _rating.join('\n'));