Start with checking the company website and Google search
Every state secretary of state has a company entity search database for (public or private) companies that are registered in that state.
J-school subscription databases
Search Google for audit reports that evaluate the company, written by government agencies or watchdogs, here are examples searching the gov domain and the org domain. You can copy the search and change the company name:
More Oversight and Regulation Databases
Use the Backgrounding People Guide to background the company executives.
Use the Guide Nonprofit Organizations - research sources for reporters to identify nonprofits associated with the company or executives.
You may be able to find undisclosed information on private companies in civil court documents.