If you make a reservation online, the cancellation policy for each hotel is clearly listed on the bottom. Also, if you've received a confirmation e-mail, the cancellation policy should also be on there. Most full-service (not resort) Marriotts that I've seen have 6pm day-of policies, but it behooves you to check the individual property.

Also, please add a "?" to your title. It was a bit misleading and I was expecting to read about you actually being charged a fee instead of a question.