To the degree that you are able to find out, I would look at average GPA at the schools you are considering, and – if possible – in your major.
At T20s the caliber of student will be relatively inelastic – they were generally high-stat, accomplished high school students who are interested in learning and contributing – so if you find meaningful differences in GPA among the schools you are considering in the major you are exploring, that may point to differences in rigor, calendar (course load/timing), curricular requirements, and grading styles/difficulty.
A small difference in average GPA may be attributable to something as simple as one prof of a very popular (or required…) course giving everyone As and Bs (inflation), or employing a curve which results in half the class receiving a C (deflation).