I had to learn what we used to call “desk checking” when I started programming, out of necessity. Desk checking is reading through your code and “running” it in your head, keeping track of state on paper. You learn to notice edge cases because at every step you have to think about the next possible states. After some practice you get better at this and it becomes more automatic.

This isn’t practical for large code bases, but you can do it with individual functions and chunks of code. I still work this way but I don’t think it’s a common technique anymore.