How do I know a page was posted back because of an ajax event in Page_load

In Visual Studio 2008 I have a asp:ScriptManager and asp:UpdatePanel on the page.

I also have an event which posts back but does not cause a full page reload (because of the ajax). Also have other buttons on the page which when pressed do cause a regular postback which refreshes the whole page. My question is: is there a way to know in page_load when it was fired because of a regular postback or because of an ajax event (kind of like ispostback which detects if page was posted back)?