How do you determine what needs included in /etc/sysupgrade.conf?

I have an old router that I want to upgrade to OpenWrt 19.03 but I was unaware of the file /etc/sysupgrade.conf (which specifies which files to preserve custom files between upgrades); how can I tell which files are custom (and need added to the sysupgrade.conf) before I blow out the configuration?

Or do I just need to do a backup of the thumbdrive before hand and do it by trail and error?

If the old router is not running 19.07.X version, then take a backup of the settings and use it as reference only to restore the configuration manually. That means that you will deselect the tickbox to Keep settings before you upgrade.

Most of the configuration files are located in /etc/config/ which is backed up. If the custom configuration files are somewhere else you'll have to add them in sysupgrade file.