The City of Baltimore keeps an up-to-date website that lists the restaurants (with addresses) that have been closed for health violations. Currently, the lists go back to June of 2006, so you can see if your favorite restaurant in Baltimore City has been closed. Violations include pest infestations, unsanitary conditions, and other violations deemed serious enough to close the venue until the violations are remedied.
Another useful source of information is a map maintained by the Baltimore Sun. The map shows you the closures by address, and it includes the same restaurants mentioned in the lists on the Baltimore City website. This is a good tool to have particularly if you are unfamiliar with a section of town and want to know what to avoid. If you want to report a restaurant or food establishment in Baltimore City, call the "311" information call center.