Depends on the jurisdiction. Like Paco says, different authorities have different definitions. Some look only at an OAT gauge, some look for ice on sponsons or wipers, some look at the torque gauge, some look at a weather forecast. In some places regardless of what is forecast, if some brave ice-dummy soul ventures up and reports back that he isn't picking up ice then to the rest of the pilots it is no longer "Known Icing", in fact it is now "Known Not To Be Icing".
Pilots in places like Norway or the Canadian North that wouldn't turn a blade from September to April following the rules of more temperate locales, have built up a vast body of knowledge based on experience. Such as Whirl's comment on snow. This is passed down from senior pilot to junior pilot. As it should be.