How do I know my system updates are trustworthy?

view story

http://askubuntu.com – I routinely update my system whenever it notifies me of software updates. This is one of those things that I just trust to work without knowing the details, but I have become curious recently: How do I know that the process checking for updates will only show legitimate updates? the updates I receive and install are not malicious? I know that I have a set of software sources that I specify myself by URL and that whether I trust those sources is my decision. But what happens once I specified those URLs? From what is common these days, I would suspect that the authenticity of those sources (HowTos)