Contact the employer's human resources department and request the Federal Tax ID Number. If you are or were an employee for the company, you should be given the number. The employer's federal tax ID number will also be on any W-2 forms you might have received from the employer.
Use the Security and Exchange Commissions EDGAR database website to find a public company's federal tax ID number. (See Resources.) All public companies have to file their financial information with the SEC, and you can use the EDGAR database website to find a public employer's federal tax ID number.
Use GuideStar to find the Employer Federal Tax ID Number for a nonprofit organization. (See Resources.) Nonprofit organizations provide their tax ID number on Form 990 and Guidestar will allow you to search for any nonprofit's Form 990.