You should tell each other your handicap before you start play in a match, but you must declare a correct handicap. If it is incorrect, and your opponent plays a stroke (or takes similar action – e.g. conceding a stroke or hole) then you will have to suffer the consequence, as follows:
- too high a handicap declared – disqualification
- too low a handicap declared – you now need to play this match off the handicap you declared
Golf Rules Guru 