You can't verify. Even if you could, you can't ensure that the code will be replaced the moment you turn around. I investigated this thoroughly a few years ago

You can't verify. Even if you could, you can't ensure that the code will be replaced the moment you turn around. I investigated this thoroughly a few years ago, and the solution I came to is distributed computing. Join a peer-to-peer network an run the code directly on your own hardware or depend on someone that you actually trust. As far as I can tell there are no shortcuts that allow you to trust strangers with sensitive data.