"PostBack is the name given to the process of submitting an ASP.NET page to the server for processing ."



protected void Page_Load(object sender, EventArgs e)
{
   if (IsPostBack)
   {
        ClientScript.RegisterHiddenField("hdnPostBack", "1");
   }
 }




$(document).ready(function () {

    var isPostBackOccured = document.getElementById('hdnPostBack');
    if (isPostBackOccured== null) alert('Page is called first Time');
    else alert('Page is called after Postback');
});