Have your dentist write you a letter of recommendation. If you shadowed for 40 hours then you put it in AADSAS when you fill out your application.
When your dentist writes you a letter of recommendation fill out a litle BIO sheet for him about yourself. Grades, extracurricular activities, hobbies, the hours you shadowed, etc. It helps him fill out your letter. Have him also mention the amount of hours you shadowed.
When admissions looks at your app they will see that you have shadowed 40 hours and they will see proof of it in your letter of rec. This makes you CREDIBLE.
You should do this for most of your letters of rec. If you tutored in math lab have your math boss write you a letter of rec. etc. (no you do not have to get a letter or rec for EVERY thing in AADSAS, just pick the best couple or so.)