Do this via CI to keep an eye on it. It's very easy to add a new file and forget.
agrun should work, but: https://github.com/ggreer/the_silver_searcher/issues/964#issuecomment-1625721711
reusetries to do too much IMHO, including enforcing a specific project structure and locale: https://github.com/fsfe/reuse-tool/pull/794
grep -rL SPDXis a nice and simple approach, but it will traverse into
fd | xargs grep -rL SPDXworks. Just need to ignore the obvious exception:
LICENCE(or rather, check that it's the only result).