Reading the code shouldn't be your last resort.
Reading the code isn't always comfortable. It can feel like an arduous chore, especially if it isn't a core habit of yours (yet). This feeling is self-inhibiting. Recognize that it is born from discomfort, not logic.
The code is truth. The only complete truth.
If physicists had all the source code for the universe, how many experiments do you think they would run? My guess is zero.
Read the code and all else will follow.
Always read the code. Even when they tell you it doesn't or shouldn't work that way. Especially when they tell you that.