It is often needed to detect that the currently displayed page is part of the admin interface.

For example, integration of Netgen Tags into Admin UI uses an event listener to inject a different pagelayout into tags admin interface based on which context it is running in (Platform UI, Admin UI, Netgen Admin UI, standalone).

A nice way to detect that current siteaccess is in fact an Admin UI siteaccess would be to put a flag in the request attributes, which can be queried at any time in custom code.