Ask your employer. They are required by law to have a Notice To Employees poster displayed somewhere in the workplace, providing you with that information. Also, if you have been injured, and have been unable to earn full wages for five (5) or more full or partial calendar days, your employer must report your injury to their insurance company and to the DIA. They are required to give you a copy of this report, which contains the name and address of the insurer. If you have tried to get this information from your employer and failed, the Office of Insurance in Boston can assist you, 617-626-5480 or 617-626-5481.
You can also find the name of the insurance company by using the "Workers' Compensation Proof of Coverage and Verification Search" web tool which is located on the DIA landing page. This Proof of Coverage Application allows the public to search workers' compensation insurance coverage information for policies in the Voluntary Market and Assigned Risk Pool.