check which control fired post back at pageload

If you want to know which controlled fired post back at page load, here is some sample code.

1) for link button:

string ctrlname = Page.Request.Params["__EVENTTARGET"];
                        if ((!string.IsNullOrWhiteSpace(ctrlname)) && ctrlname.EndsWith("lnkSelect"))
                        {
                            LinkButton lnk1 = Page.FindControl(ctrlname) as LinkButton;
                         }

2) for Image button and Button:

protected void Page_Load(object sender, EventArgs e)

    {

        if (IsPostBack)

            Response.Write(getPostBackControlName());

    }

   

    private string getPostBackControlName()

    {

        Control control = null;

        //first we will check the "__EVENTTARGET" because if post back made by       the controls

        //which used "_doPostBack" function also available in Request.Form collection.

        string ctrlname = Page.Request.Params["__EVENTTARGET"];

        if (ctrlname != null && ctrlname != String.Empty)

        {

            control = Page.FindControl(ctrlname);

        }

        // if __EVENTTARGET is null, the control is a button type and we need to

        // iterate over the form collection to find it

        else

        {

            string ctrlStr = String.Empty;

            Control c = null;

            foreach (string ctl in Page.Request.Form)

            {

                //handle ImageButton they having an additional "quasi-property" in their Id which identifies

                //mouse x and y coordinates

                if (ctl.EndsWith(".x") || ctl.EndsWith(".y"))

                {

                    ctrlStr = ctl.Substring(0, ctl.Length - 2);

                    c = Page.FindControl(ctrlStr);

                }

                else

                {

                    c = Page.FindControl(ctl);

                }

                if (c is System.Web.UI.WebControls.Button ||

                         c is System.Web.UI.WebControls.ImageButton)

                {

                    control = c;

                    break;

                }

            }

        }

        return control.ID;

    }

}

Source:
Server postback link 1

server postback link 2