Referees are required by USSF to re-certify yearly. One exception is for referees who first register in the fall. These referees are certified through the end of the following year. The best way to tell if you have to re-certify or not is to look at your referee badge. What year is on the badge? You are certified through the end of the year listed on the badge.
As an alternative, check the date on your US Soccer Federation Referee Registration card.