Go to USPS.com and choose the button on the upper left hand side that says "Find a Zip Code."
Enter the address, city and state of the property's postal code you are looking for. Hit "Submit." The results will display the official mailing address. The post office website requires an exact match to return results. If you do not know the exact address of the property you are researching, it is best to use local government records.
Go to your counties tax assessor or auditor's website if you don't know the exact address of the property for which you want to find a postal code. Most property searches on these websites return broad results if an exact address is not given. For example, when you go to the Franklin County auditor's property search and enter the word "Main" into the search field, all properties with the word "Main" in their address will be returned. The search results return the legal addresses of the properties, and these can be used to determine postal codes by following the steps above.