When you add some feature to NixOS, you should write a test for it. NixOS tests are kept in the directory <filename
xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/tests">nixos/tests</filename>, and are executed (using Nix) by a testing framework that automatically starts one or more virtual machines containing the NixOS system(s) required for the test.