Some NHS and private dentists are failing to carry out even basic examinations, according to Which?
The consumer organisation carried out a snapshot undercover investigation of 20 dentists, 10 private and 10 NHS, in which 11 visits were rated as "poor" or "very poor" overall.
Researchers found examples of dentists rushing examinations, overlooking vital checks and developing inappropriate treatment plans.
Five visits lasted less than 10 minutes, with two dentists spending just five minutes with their new patients, and on five visits, X-rays were not offered.
It should be standard practice to X-ray a new patient's teeth to check for problems such as tooth decay, unless there are health or other reasons that preclude it, a spokesman for Which? said.
Checks on the soft tissue inside the mouth, important to screen for oral cancers and other medical conditions, were reported on only five of the 20 visits.
Researchers also noted that only three of the 20 dentists examined the face and neck, necessary to check for swollen glands.
A Department of Health spokesman said: "Poor care of any kind is unacceptable.
"While this survey only covers a very small number of practices, the findings will be considered by the General Dental Council, who set and monitor professional standards for both NHS and private dentists.
"The proposed new dental contract will ensure dentists are providing the highest quality care for their patients. But dentists still have an individual responsibility to meet professional standards whether practising in the NHS or privately."