How can I find out if a doctor has been sued for malpractice?

Check with the local state medical board to determine malpractice record of your physician. Remember, anyone can sue anyone for anything. Each malpractice suit should be evaulated for obvious merit. 

Information on a doctor’s malpractice history is available in a certain number of states, such as New York, Massachusetts, New Jersey and Connecticut. There is strong public and sometimes political pressure to make that information more available to the public.

It is almost impossible to find out if a doctor has been sued for malpractice until you have sued him or her.  There are about four indirect methods you can try, but the best way is to act the doctor directly.  If the doctor answers frankly and without taking offense, that tells you a lot about his or her character.  If the doctor refuses to answer, acts offended, is evasive or shifty or tries to make excuses, that tells you even more.  People are not accustomed to asking this question of their doctors and you may find it difficult at first, but this is the only really effective method.
Indirect ways include Googling the doctor by name, checking with your state's licensing board to see if they keep a public database (most states do not), and checking with the courthouse in the town where the doctor practices.
There is a nation-wide government database (the National Practitioner Data Bank) of physicians who have paid a malpractice claim, but studies have shown that many doctors' malpractice claims are not reported to the NPDB, even though there are fines for not reporting.  Moreover, the information is not public and the only way you can get it is to ask the doctor to give you his NPDB report.