Wood stoves have been approved for use in Massachusetts if they have a UL approval number 1482, which is located on a metal tag riveted to its side. Pellet stoves should have a similar metal tag, but should bear an Americans Standard Testing Method number 1509 on it.
Also said factory-built fireplaces, which are rare, but occasionally purchased by residents should have a UL number 127 on them.
The Official Website of the Executive Office of Public Safety and Security has a list of general requirements along with frequently asked questions regarding wood-burning stoves and other solid fuel-burning heating appliances.