You can determine this from the SourceSource control property of the Context menu during the opening event.
This is the VB.net code, haven't checked for the C# equiv.
Private Sub ContextMenuStrip1_Opening(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles ContextMenuStrip1.Opening
Debug.WriteLine(ContextMenuStrip1.SourceControl.ToString)
End Sub