What are you trying to do? If you are developing a Provider, Outlook tells
you if you may display dialogs.
I don't think checking for the explorer is a good idea. Even if you're
writing a service (which has no GUI) then you'll find a background explorer
running.
Can you start Outlook without logging in? (I don't think so, but I'm not
sure.)
Oh, sorry, you were talking about the Outlook Explorer object. I don't know
that object model very well and thought you wanted to check for an existing
Windows Explorer to determine if there's a GUI.
-> Take the answer from Dmitry. His answers are more worth than any
Microsoft documentation.