Hi All,
I like to add a field in the user authentication table id'ing some level of access (i.e. admin, guest) I place this in a session variable and check for both a sessionID and the level of access in the pages similiar to Ben's method. To make this even more secure you could encrypt the level in the Dd by concatenting it with a secret string.
One other option is separate out the admin files from the main files by placing them in a separate folder and limit the access to the folder.
hth
Bastien