Assuming that you do not have your USSF Referee Card and have not hung onto and filed away your past registration forms, the USSF database is the best source from which to obtain this information.
You are probably aware that the USSF maintains a comprehensive database of all of its referees. However, access to it is restricted. Thus you will need to contact someone within your state administration who either has been granted access or who can direct you to a person who does have access or you will have to contact the USSF national office in Chicago directly. It is best to try to go through your state personnel first.