HomeIn Washington, any person attempting to purchase alcohol is required to present proper identification proving they are 21 or older.  If you have doubts about the person's age, even if the ID is one of the acceptable forms of ID, you can ask the customer to complete a Licensee Certification card.

Licensee Certification cards must be filled out completely and signed by the person presenting the ID.  These cards must also be filed by the close of business the same day the cards are completed.  Licensee Certification cards must be available for examination by any law enforcement officer.

A completed Licensee Certification card is a defense if you are charged with serving alcohol to a minor (if the card is filled out correctly and the minor has signed the card) and is evidence that you have acted in good faith to prevent the sale of alcohol to a minor.
A sample Licensee Certification card is provided here.