true, --substitute false will compile all dependencies, down to the compiler itself, but a simple (/s)

nix-build "  " [package] --check

will compile just the chosen package, skipping dependencies, and compare it against the cached binary in the repo to ensure they're equivalent.

I could have gotten that nix-build command slightly off as I'm typing this from memory. I am also saying most of this in jest as they aren't really solutions to anything mentioned above and I moreso find them interesting features.