var fso,s=filespec;   // filespec="C:/path/myfile.txt"
fso=new ActiveXObject("Scripting.FileSystemObject");
if(fso.FileExists(filespec))
s+=" exists.";
else
s+=" doesn't exist.";
alert(s);

var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
xmlhttp.open("GET",yourFileURL,false);
xmlhttp.send();
if(xmlhttp.readyState==4){  
if(xmlhttp.status==200)s+=" exists."; //url存在  
else if(xmlhttp.status==404)s+=" doesn't exist."; //url不存在  
else s+="";//其他狀態  
}
alert(s);