In the United States, all non-profits recognized by the Internal Revenue Service (IRS) are assigned 501(c)(3) status, which legitimizes their tax-exemption. Officially known as the Employer Identification Number, a non-profit's 501(c)(3) number can be found on their 990 form, which non-profits file every year. The 990s are public information, and the website GuideStar offers a unique service of cataloging 990s and providing access to them free of charge. Although you'll be able to search their catalog the moment you arrive at the site, in order to actually view 990s, you'll need to first register and login.
Go to GuideStar's website. If you don't already have an account, click the "register" button and fill out the required information. Then log in by entering your email address and password.
Enter the name of the non-profit you wish to look up in the search bar. A screen will load listing all relevant results. In some cases, an organization might be registered under a different name than the one they are publicly known by. GuideStar recognizes this and still lists those organizations when search for by their unofficial name. They will be listed under their official name, but their unofficial name will also be listed in an "Also Known As" field.
Click on the organization you wish to get the tax ID about. This will load a page dedicated to that non-profit with everything from contact information to the organization's mission statement. Click the "Forms 990s and Docs" tab to access their 990 forms. These come in PDF format.
Open the most recent 990 PDF. For some organizations, this can take a while, as their tax forms are hundreds of pages long. For smaller organizations, the form might only be a few dozen pages and will load much more quickly.
Find the entry beneath "Employer Identification Number." This is their tax ID number which defines them as a 501(c)(3). It is listed near the top of the first page on the right-hand side.