Just read every line of the generated code and make sure it is as clear and good as possible. If you can't understand it when it's new you won't tomorrow, either. This verification process places a natural limit on the rate at which you can safely generate code. I suppose you could reduce that to spot checks and achieve probabilistic correctness but I would not venture there for things that matter.

Because lines of code interact with each other. Understanding what one line does in isolation does not always show the rough edges that are found when code interacts. The challenge is seeing the forest instead of individual trees.