Re: trusting a dentist?
Of course, the hope is that each dentist is doing their utmost to keep their operatories as sanitary as possible. There are sterilization laws to be followed that are not taken lightly. Where I work, even the arms of the light are sterilized between patients. It is unreasonable to expect a dentist to change their gloves every time they readjust their light.
To me, it's like eating in a restaurant. You hope that the food preparers wear gloves. You hope they wash their hands before handling any food. You hope that your silverware is clean from the person who ate there before you. If there are blatant signs there that are giving you a red flag, then of course you need to honor that. But otherwise, you assume the best. I think the same goes for going to the dentist and the doctor.