To do what you're describing means that you would be opening the same page in the window as the "parent" page, correct? If that's the case, then it's pretty simple to know whether or not that page is in a window or not, because the window will add "rwndrnd=some long number" to the querystring of the page that is open in the window.
So, in you're Page_Load event, or some other event that is controlling what to do when the controls are instantiated:
If Not IsPostBack Then
If Request.QueryString("rwndrnd") <> ""
' The page is opened in a RadWindow
Else
' The page is opened in the browser
End If
End If