title: Determine if user can access the requested page?
body: <p>I have an ASP.Net website with multiple roles, each with access to a separate directory (i.e. admin users can access /admin, shoppers can access /shop etc), using a shared login  page. If someone visits the login page with the return URL set to a directory they do not have access to (e.g. a shopper visits /login.aspx?returnurl=/admin/index.aspx), the user can authentice successfully (the login credentials are valid), but they end up back at the login page (they don't have access to the page they've requested).</p>
  
  <p>How do I pick this up, so I can display a message do the user?</p>