I’ve found the nox-update command in GitHub - madjar/nox: Tools to make nix nicer to use to be really useful for this. It produces a lot of output when comparing two entire systems, so I usually run it like this to get a higher-level summary:
nox-update --quiet /run/current-system result | \
grep -v '\.drv : $' | \
sed 's|^ */nix/store/[a-z0-9]*-||' | \
sort -u