Are you shadowing and getting credit at your school? Or just contacted the dentist. I did both. I got credit hours for school where I had to write a journal, and I also did it independently.
Independently I would suggest keeping a log of all your hours, and then toward the end of your shadowing experiences you might want to ask for a recommendation. Many schools require a recommendation from a dentists, and others suggest it. Often in the recommendation the dentist will write something to the affect that you completed over "blank " many of hours in his practice.