Every state keeps records of incorporation information -- including the date of incorporation -- for companies that incorporated there. You can find a company's date of incorporation by searching the appropriate state website, looking on the company's investor relations web page or using a legal database.
Every corporation must file articles of incorporation with its state corporate filing office before beginning business activity. This document contains basic information about the business, such as the corporate name, address, the names of the officers, the purpose of the business and information about corporate stock. The date that these articles are filed is referred to as the date of incorporation.
Once the articles of incorporation are filed with the state, information is available to the public. If you know what state the corporation incorporated in, you can find information about the corporation from the state's division of corporations.
Many corporations choose to incorporate in Delaware because of its significant legal and tax advantages vs. other states. Delaware lets you find dates of incorporation by searching the entity name at the Division of Corporations' search website. For example, if you type "Disney" into the search field and click on "DISNEY/ABC INTERNATIONAL TELEVISION, INC.," you'll see that the company incorporated on December 3, 1985.
Publicly traded companies publish incorporation information on the Investor Relations sections of their corporate websites. For example, Google publishes all amendments and restatements to its certificate of incorporation in PDF format. If you look at its fourth amendment, which is hosted on Google's IR site, you can see that the original date of incorporation was October 22, 2002. Johnson & Johnson also makes its Certificate of Incorporation publicly available.
Many legal databases, such as CCH IntelliConnect, allow users to search the incorporation information of different companies. However, users typically have to pay for the right to use these research databases. Many universities, such as Harvard Business School and University of San Diego Law School offer students and faculty access to CCH IntelliConnect and other legal databases for free.