how to find which packages are installed system-wide in NixOS?

There's no specific tool for this. You may like the system.copySystemConfiguration option (see the docs for "caveats").

You'll get relatively close with nix-store -q --references /run/current-system/sw – the list of nix store paths directly contained in systemPackages, but note that various NixOS options may add packages in there.

Due to the approach NixOS takes on installing software, there's no such thing as a package which is installed system-wide. Not in the same sense of most package managers, that is.

However, if you treat a user environment (aka. profile) as the equivalent of system-wide, then you can use nix-store -q -R /nix/var/nix/profiles/system-${n}-link to list the dependencies of a given user environment; where ${n} is the profile's generation number. By dependencies, I mean direct and in-direct dependencies.

In addition, if you use the --tree instead of -R argument you can get an ASCII dependency tree.