The browser object model doesn't provide access to these headers but in IE 5 and newer you have an ActiveX Object Microsoft.XMLDOM which can request the page and return the headers as shown below:

<script language="JavaScript" type="text/javascript">
<!--
var http = new ActiveXObject('Microsoft.XMLHTTP');

// display headers of current document:
http.open('HEAD', location.href, false);
http.send();
alert(http.getAllResponseHeaders())

// or display headers of yahoo.com:
http.open('HEAD', 'http://www.yahoo.com/', false);
http.send();
alert(http.getAllResponseHeaders())

//-->
</script>