How do we know Open Source software is using the code it says it uses?

This might be a dumb question, but when something is "open source" it means the source code is visible. But when you download something open source, unless you choose to go through the effort to compile it yourself, you are downloading an already put together piece of software. What's to stop a developer with malicious intent from sneaking some code into the compiled version that isn't in the source code on display? Is there a way to verify this other …

code open source privacy software