To find out if a lawyer has been disbarred or formally disciplined, call a state supreme court clerk, and provide him with the lawyer's name. Another option is to go online and access attorney records from state association websites.
When speaking to a state supreme court clerk, ask about any charges that have been filed against the lawyer in question. In addition, ask for a disciplinary record so you can research any problems the lawyer has faced in the past. Each state keeps this information in the public record and is obligated to divulge it when asked. Keep in mind, however, that unresolved complaints are kept private until the state board has ruled on the matter. This means the lawyer in question could be facing disciplinary action, but that it is not yet available in the public record.
You can find this information online as well, since many state bar groups keep attorney records online. Search the state bar website. Keep the attorney's street address on hand, since there is often more than one attorney with the same name state-wide. The state bar website has information about complaints against attorneys and their outcomes, including whether an attorney has been disbarred.