A human brain is a big pile of jellied meat spread. An LLM is a big pile of weights strung together by matrix math. Neither looks "intelligent". Neither is interpretable. The most reliable way we have to compare the two is by comparing the outputs.
You can't drill a hole in one of those and see something that makes you go "oh, it's this one, this one is the Real Intelligence, the other is fake". No easy out for you. You'll have to do it the hard way.