Every state has a Secretary of State or other state agency that's responsible for business entity filings. In most states, the website of this filing agency includes an online business search tool that allows you to search for a business by name. The Secretary of State search results will include company details such as the initial filing date, formation state, entity type, primary address, mailing address, and registered agent.
This information will also be located on your company's Articles of Incorporation. Depending on the state, this document may also be called a Certificate of Incorporation, Certificate of Formation, or Articles of Organization.