One of the best ways to check for malpractice suits against a dentist is to look him up with his state's dental board. The dental board also has information about complaints that have not progressed to lawsuits.
Each states has a dental board that regulates dental licenses and oversees complaints against its members. Dentists are required to be licensed with this board in the state in which they practice. If your prospective dentist does not have a current license with the state board, find another dentist and report the unlicensed one to the board.
The state dental board is generally the first place people go to make a complaint, even if they intend to file lawsuits later. Initial complaints are usually kept confidential until the board investigates them. However, if the board finds that they have merit, the information is sometimes made public. Dental boards sometimes have online listings of accusations against their members. If your state does not, call the dental board to ask for specific ways to look up malpractice complaints.
You can also try checking court records at the courthouse that serves the area in which the dentist practices. Search these for the dentist's name. Another method that may be less reliable but might provide valuable information, is to ask the dentist directly. If the dentist refuses to answer or makes you uncomfortable, consider looking for a different dentist.