Determine How ASP.NET Web Pages Were Invoked

It is often useful to know how an ASP.NET Web page was invoked: whether by an original request (an HTTP GET), a postback (an HTTP POST), a cross-page post from another page (an HTTP POST), or a transfer from another page using the Transfer method or using a callback from the browser. The Page class exposes a set of properties that you can use to determine how a page was invoked.

To determine how an ASP.NET Web page was invoked
• Examine the values of the following Page class properties, and then refer the table to determine how the page was invoked:
o IsPostBack
o PreviousPage
o IsCrossPagePostBack
o IsCallback

Original request
• IsPostBack is set to false.
• PreviousPage is set to null (Nothing in Visual Basic).
• IsCallback is set to false.
Postback
• IsPostBack is set to true.
• PreviousPage is set to null (Nothing in Visual Basic).
• IsCallback is set to false.
Cross-page posting
• IsPostBack is set to false.
• PreviousPage references the source page.
• IsCrossPagePostBack is set to true.
• IsCallback is set to false.
Server transfer
• IsPostBack is set to false.
• PreviousPage references the source page.
• IsCrossPagePostBack that is referenced in the PreviousPage is set to false.
• IsCallback is set to false.
Callback
• IsPostBack is set to false.
• PreviousPage is set to null (Nothing in Visual Basic).
• IsCallback is set to true.

source: http://msdn.microsoft.com/en-us/library/ms178141.aspx