Found it...
It was kind of tricky because header names that are not passed via the request are not populated in the Enumeration object. Here is the code I used to list out the headers from the request.
Enumeration e = request.getHeaderNames();
String name = "";
while (e.hasMoreElements()) {
name = (String) e.nextElement();
System.out.println(name + ": " + request.getHeader(name));
}
... and here is how to get the header that returns the link back URL.
request.getHeader("referer")