How is consent proved (or the lack of it) in dental practice?

I understand that "informed consent" is required for all dental procedures. That is, the dentist shall ensure that the patient is supposed to be informed of the procedures he will perform, the risks and the cost. But in practice, at least for minor procedures, there is rarely a consent form signed. So, when is it considered that the patient gave consent in case a conflict arises? Who takes the risk when this is not done formally or informally with proper information? For example, a patient goes for a regular cleaning and the dentist bills a "full mouth debridement" which has double the cost and is not covered by insurance, without that been clearly explained. Can the patient claim he never gave consent? (and win in court with that argument if he gets to that point) All responses welcome, but please indicate if yours is just an opinion or if you have real knowledge from the dental or legal perspective.
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I sign a consent form prior to all procedures in both my general dentist's and my periodontist's office. All prospective procedures are explained to me in advance. Find another dentist who has decent ethics.
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