It depends to some extent on the class of competition but mostly on common sense. Recognise that they'll never attain perfection but don't let your standards get so sloppy that you excuse extreme violations so as not to discourage a youngster. When it is necessary to call an infraction, be sure that it is explained to the swimmer so that they will know that they have been judged fairly. Some officials adopt a punitive attitude, apparently to assert their own authority or in a misguided effort to show their superior knowledge. They may be quite well informed about swimming techniques and rules but if they are more concerned with disqualifying than assuring fair competition, they should not be placed in a position where they can call infractions.
General conduct is most important.
Smoking is taboo for swimmers and food and drink are generally prohibited in well regulated swimming areas. Pay attention to your official job. Stop talking during starts and while announcements are being made. You have a responsibility to control the behaviour of the members of the Club with which you are associated, both on the deck and in the locker room. If others neglect to do so, show any reasonable authority needed to assure good competitive conditions and to safeguard the facilities and the property of others.