get submit button id in Page_Load

The sender argument to the handler contains a reference to the control which raised the event.

    private void MyClickEventHandler(object sender, EventArgs e)
    {
         Button theButton = (Button)sender;
         ...
    }
 

Edit: Wait, in the Load event? That's a little tricker. One thing I can think of is this: The Request's Form collection will contain a key/value for the submitting button, but not for the others. So you can do something like:

   protected void Page_Load(object sender, EventArgs e)
  {
       Button theButton = null;
       if (Request.Form.AllKeys.Contains("button1"))
           theButton = button1;
       else if (Request.Form.AllKeys.Contains("button2"))
           theButton = button2;
       ...
   }
 

Not very elegant, but you get the idea..