Well, I am not sure there is a set list of things you should expect to see in a LOR... What I would suggest, if you have time before you apply, is to join your universities PPAC (Pre-professional advisory committee) and have them write one for you. They will interview you and have you get a LOR from 1 professor and 1 dentist and package all of it together. I did this, and elected not to read my LOR before sending it out (obviously, it can seem suspicious to schools if you are screening your letters before approving them).
If you don't have time for PPAC, then make sure the dentist you shadow likes you, because I don't know many that would let you tell them what to write. If your writer likes you and genuinely wants you to get accepted, he will write you a good LOR.