The United States Postal Service (USPS) has a useful website for looking up Zip Codes. To look up the Zip Code, you will need the street address (including house number), city/town, and state. Click here to access the USPS Zip Code lookup service.
Zip Codes can change over time, so it is a good idea to verify your last U.S. residence Zip Code on the USPS website, as well as to obtain the full "ZIP+4" code.