This answer may be specific for SharePoint 2010:
On the webpart page, your page viewer part will contain a div element with a unique id (e.g. WebPartWPQ3). Inside this div is an iframe that points to the page to display (i.e. src="your page"). You can use your favorite DOM inspector to find this div and get the id.
var pg = $("#WebPartWPQ3 iframe");
Forgive me for not testing the above snippet (my SharePoint box is out of commision at the moment). But the jist of the above should work.