High schools provide a profile to the colleges. The profile gives enough information that the colleges get a good idea of the rigor of courses.

The profile might include the courses offered, grading system, GPA/class rank correlation, average SAT scores broken down by GPA range, colleges attended by the prior graduating class by GPA range, etc.

Looking at a past profile for our district, the 40 kids in the top 12% GPA had an average SAT score of 2086. The bottom 20% had an average SAT score of 1484. 91% of students that took AP tests scored 3 or higher.

From that information you can draw some conclusions regarding the rigor of AP/Honors courses at the school. I think most colleges have an idea of the rigor of certain high schools if they get multiple applicants from the HS each year.