The three children could neither read nor write. As they are being taken away in the magistrate's car, thinking they are going to the Cova, Lucia spots a road sign for the next town. She immediately protests that they are going in the wrong direction. How would she know what was on the sign if she couldn't read?
shareTo rationalize for the film version, she wasn't necessarily reading it. She could have been on that road before, might even have gone to Ourem with her dad, and seen the sign as a landmark.
In real life, they were in a wagon at that point, not the car. What tipped her off was when the Admin. actually turned off the road to the Cova and said "The church father in Ourem wants to see you, it won't take long, I have my car there and I can get you back to the Cova in plenty of time."
I'm all right, I'm alllll right!