I am contemplating legal action. How do I find a company or partnership, or their assets?

Companies that use the surname of the individual, and partnerships can be located in the town in which they do business; companies that use other names, commonly called "d/b/a" can be located in the town in which they do business by contacting the Town Clerk; and corporations can be located at the office of the Secretary of the Commonwealth.
 
UCC filings, to protect lendors, can be found in the office of the Town Clerk and at the Registry of Deeds, as well as at the Secretary of the Commonwealth.