I need to have a hosting company upgrade the Debian OS install on a VPS. But I also need to know things like what MySQL or Perl modules were added to the server by other admins prior to me outside the /home directory. I don't have any documentation on it at all. If I don't preserve custom stuff like that, it could result in a dead website. Anyone got any tricks for figuring out what was added and when?

Why do you need it?
Your hostingcompany should take a complete backup of the machine prior to the upgrade > perform the upgrade > if Website breaks, check why and try to fix, otherwhise restore.

@Linux Mainly, because I want to limit the number of times I have to do a cycle of troubleshooting. "I only want to do this twice" kind of stuff. Once would be best. Plus, I anticipate having to do this late at night and to babysit this particular hosting company as they do it. They've told me that they're not responsible for anything that happened to the server outside of the /home directory. And, my past interactions with their support team have led me to feel paranoid that something will be missed.

@stackodev

Well, are you able to migrate the Website to another host just as a test?

@Linux Good point. I'll add it to my list of questions to my hosting company. It should be relatively simple to set up the new VPS with the site, switch the DNS, see if it works, and if not, switch it back. If it fails, I'd still have to figure out WHY it didn't work. And, if there's "deeper" stuff that doesn't work (like a forum feature missing, but that isn't easily apparent when spot-check testing), that's a risk.

@stackodev

Well, just change your hostfile so you dont need to change the DNS :)