Trust is a broad term. If you're paranoid, find the package you care about here, and read every line:

https://github.com/NixOS/nixpkgs

If you're slightly less paranoid, check the git blame logs for anyone that's touched a package you care about. If you trust all of them, then you're good.

If you're less paranoid than that, assume that someone reasonable is in charge of that repo. You'll get warnings about insecure packages. I've had to Ok a few insecure packages in my configuration.nix, because I assume the packagers are reasonable people. I may yet find out I've made a mistake.

Broadly speaking, I think it's the same model as any other distro. Debian for example has volunteers that package stuff. You can go through the same process above and decide how paranoid you want to be for that as well.