There's a tool available that can help:
Authentication and Access Control Diagnostics 1.0 (more commonly known as AuthDiag) is a tool released by Microsoft aimed at aiding IT professionals and developers at more effectively finding the source of authentication and authorization failures.
These users have often seen behavior from Internet Information Services (IIS) that doesn't seem appropriate or random when users authenticate to the IIS server. The complex world of authentication types and the various levels of security permissions necessary to allow a user to access the server causes many hours of labor for those tasked with troubleshooting these problems.
AuthDiag 1.0 offers a robust tool that offers a efficient method for troubleshooting authentication on IIS 5.x and 6.0. It will analyze metabase configuration and system-wide policies and warn users of possible points of failure and guide them to resolving the problem. AuthDiag 1.0 also includes a robust monitoring tool called AuthMon designed at capturing a snapshot of the problem while it occurs in real-time. AuthMon is robust and specially designed for IIS servers removing any information not pertinent to the authentication or authorization process.