It's pretty clear Mary learns something new upon viewing the red apple and it's easy to create an experiment for this. We'll continue using Mary's monochrome example as a basis.

Consider the following experimental setup: Mary is prepared with all the knowledge about colors and apples and every other odd thing just as before. Mary is then instructed to view an apple and then subsequently asked to follow the instruction printed in red in the next room.

In the next room there are two sets of instructions. The instructions in red say "Exit through the left door" and the instructions in blue read "Exit through the right door". Behind the left door is a million bucks and behind the right door is nothing.

Does Mary get the million bucks? Would Mary get the million bucks had she skipped looking at the apple? It's probably helpful to do this with several Marys and ask, is there evidence that our Marys choose the left door greater than the expected probability attributed to random chance? If we can answer yes, no, and yes, then we can infer that choosing the view the apple changed Mary's mental state and that yes, she did learn something new that helped her to identify the correct text.