One of the most common tools I recommend on forums.iis.net is Process Monitor by Sysinternal.
The tool is used for stroubleshooting permission related issues, this could include Registry,
Folder or File. This article shows how to use process monitor to filter a specific IIS
(w3wp.exe) process, capture the user property. I wish process monitor included
the user property by default. Using Process monitor can help rule out
permission issues. One of the things you’ll need to do before using is browse
your site, reproduce the issue. This will warm up the w3wp process.
PS:When using this process to troubleshoot UNC based issues, I would run process monitor
on both the Front-end, and UNC server (assuming it’s a windows server)