Can I install packages built from source only

In Debian there is main archive area that all packages are built from source. However, in nixpkgs some FOSS packages are just a wrapper of binaries downloaded from Internet. Is there any way to know if a package is built from source without looking into the source code? Can I install packages that are built from source only? Is there any way to ensure the package can be built from source offline?

There is an RFC which should address some aspects of this: [RFC 0089] Collect non-source package meta attribute by risicle · Pull Request #89 · NixOS/rfcs · GitHub

Thanks! Good to know there has been an RFC!