Check the expiration date on your passport, which is listed on the information page of your passport. The date listed as the expiration date is non-negotiable and if you've passed that date your passport is not valid. In addition, many international countries do not accept passports that are within six months of their expiration date, which means if you're traveling internationally in January and your passport expires in June you need to renew it before your January trip.
Compare the name on your passport with the name on your driver's license. If you were recently married or divorced or legally changed your name for any reason but neglected to renew your passport to state your new name your passport is no longer valid.
Check your passport for any damage. Normal wear and tear is expected by customs agents on U.S. passports but excessive damage could cause your passport to become invalid. Excessive damage includes water damage, missing pages, rips, tears and other things that impair the state of your passport.