"just know" is long-term memory. That's the technical, correct term, and is exactly what it says.

And at expert levels it is what separates the truly good programmer from the novice, and even the "talented" young. Realizing this, as long as you keep learning, no fear. The only price is continual practice: effort and time.

That also means be constantly self-aware. If things are going too smoothly, it means it's time to pick up something new.