What I settled on was to test if /nix/store is owned by root with the sticky bit. There’s a long explanatory comment in this code
# Setup Nix
# We need to distinguish between single-user and multi-user installs.
# This is difficult because there's no official way to do this.
# We could look for the presence of /nix/var/nix/daemon-socket/socket but this will fail if the
# daemon hasn't started yet. /nix/var/nix/daemon-socket will exist if the daemon has ever run, but
# I don't think there's any protection against accidentally running `nix-daemon` as a user.
# We also can't just look for /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh because
# older single-user installs used the default profile instead of a per-user profile.
# We can still check for it first, because all multi-user installs should have it, and so if it's
# not present that's a pretty big indicator that this is a single-user install. If it does exist,
# we still need to verify the install type. To that end we'll look for a root owner and sticky bit
# on /nix/store. Multi-user installs set both, single-user installs don't. It's certainly possible
# someone could do a single-user install as root and then manually set the sticky bit but that
# would be extremely unusual.
set -l nix_profile_path /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
set -l single_user_profile_path ~/.nix-profile/etc/profile.d/nix.sh
if test -e $nix_profile_path
# The path exists. Double-check that this is a multi-user install.
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Also to note, checking for the nixbld group won’t work if a multi-user install was replaced with a single-user install without fully cleaning it up first. This is true of testing pretty much anything outside of /nix/store.
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