Review your paycheck stub, if available. Occasionally, the employer's number can be located there.
Review your W-2 Internal Revenue Service tax form. The employer's identification number is located on the line that states "Employer ID." If you can't find your W-2 from a previous year within the past 10 years, the IRS can provide you with a copy of your tax return -- which will include your W-2 information -- upon request, although it charges a fee for this service. Find a link in References.
Review any previous tax filings that contain reported earnings from the particular employer of which you are seeking the number. Your tax filings should contain the name, address and identification number of the employer.
Contact the employer and request the information directly. This request may have to be in writing, in which case you should request from the employer exactly what information you desire to have.
Consider utilizing an online private search company to locate the number on your behalf if none of your other attempts bear fruit. This company will charge you a fee for conducting the search.