Called By Ajax

My web site HTML comes from a database via querystring e.g. page.asp?pid=55. I have a separate asp search page, search.asp. Its output is retrieved by an ajax call from page.asp and its HTML is then neatly wrapped and styled by page.asp.

I want search.asp to recognise if it is not being called via ajax frompage.asp, but directly e.g. via the address bar (and if so, redirect to page.asp). Do I need to put something into the posted string, or (better) is there an asp method that can do this?

The ajax call is
function GetSearchPage(st) {
var poststr = "searchterm=" + encodeURI(st);
makePOSTRequest('/search/search.asp', poststr,'divFullSearchDisplay');
}