Ask HN: What makes you trust a codebase?

I sometimes find it hard to trust other people's code. For example, recently I wanted to know how configuration file affects request processing in nginx. That was no fun at all. Software engineers trust nginx and deploy it a lot, yet they probably wouldn't be able to chart out the internal structure of it off the cuff.

It is said that many eyeballs eyeballing the code over time makes it more trustworthy. But do these eyeballs look for that inspires trust?

(I'm not meaning to single out nginx, it was just at the top of my mind as an example.)