SYSK 354: How To Determine If The Page_Load Event Is Triggered by a Standard Page Postback or by the UpdatePanel?
Ok, so you’re in a Page_Load code block, and the this.Page.IsPostBack returns true. How do you know if the request is due to a “standard” page postback or as a result of an UpdatePanel triggered postback resulting in a partial page update?
The answer is simple: when the postback is caused by a trigger for the UpdatePanel control or when you explicitly call the UpdatePanel control's Update() method, the this.ScriptManager1.IsInAsyncPostBack property will return true; whereas during a “normal” postback that value will be false.
Note: in both cases the this.Page.IsAsync will be false.