In the general case it’s impossible, as /home/jb/.config/syncthing/config.xml and /srv/export/home/jb/.config/syncthing/config.xml, may be the same file, we may be using the first as config and syncing the latter.
In another sense, we could have a marker, i.e. issue #1114.
In the meantime, a simple “It looks like you may be syncing your config directory read-write. This is usually not recommended.” warning may do the trick for the time being.
But 99.99% of the people who hit the issue aren’t doing something like that - they’re people who simply decided to share their home dir, not people who are setting up multiple mounts containing their home dir.