How to validate a third party's corporate identity?

To validate a third party's corporate identity, you need to find their state of incorporation. Start with their mailing address for their corporate headquarters. The location of the corporate headquarters is not necessarily the state of incorporation.You can search the secretary of state office for the state of their headquarters. For example, if their main office is in San Francisco, California, then you would search in the Secretary of State for California. The Secretary of State for California should have on file either articles of incorporation or a registration as a foreign corporation for the company you are researching. In this case a "foreign corporation" does not mean outside the United States. A foreign corporation is a company registered or incorporated in another state.So the company with headquarters in San Francisco might be a Delaware corporation doing business in San Francisco. When you find the registration as a foreign corporation in California you can then find the articles of incorporation in Delaware. Please note that many Secretary of State offices charge fees for searches and documents. Practices very across states. I have included a couple example links to secretaries of state just for your reference.

Delaware Division of Corporations
California Secretary of State