ASDOH requires you to state your volunteering bosses and hours in your supplemental application. Likewise Tufts requires you to have a LOR with 30 hrs. If they check up, I'm not sure.
Tufts asks for a letter from a dentist verifying you shadowed for 30 hours. I didn't end up getting this letter in until after my interview, however, and it was fine.
I think I remember Ohio State asking me to send in a sheet with the dentists signature and number of hours observed. None of the other 11 schools I applied to asked for anything.
Shadowing is more for us to know dentistry is right for us. I don't think schools care too much for it and hence won't really to get proof you actually did it.
Schools could easily call up the office you shadowed in though. 😀