What is the best way to troubleshoot 500 internal server errors from BIG-IP or client?

I am using a single virtual server with a very vanilla LTM policy with no modifications to lan or wan optimized TCP profiles, no changes to HTTP profile, oneconnect profile used and a access policy for certificate based authentication and SSO using KCD. There is one application with the name of https://app.com with multiple URI's (https://app.com/uri1 https://app.com/uri2 and so on. I am able to authenticate successfully and access all other URI's within this application though for a single URI I am receiving the 500 internal server error.

When accessing the same site by going directly to the webserver, I am able to access the URI that I was previously unable to access due to the 500 internal server error. I do not have access to the virtual server config at this point in time so I am unable to provide the config.

I was curious from the community what is the best way or most common way to identify what is causing this (e.g. fiddler, httpwatch, tcpdump, app server logs, etc.)