Labelling regulations are always changing and vary around the world. However in most countries it is obligatory to state on the label that the wine contains sulfites or contains sulphur dioxide, if the level exceeds 10mg/l. (The term sulfites is used as an inclusive term for sulphur dioxide and sulfite ions). This level means that virtually all wine needs to be labelled as such. Sulphur Dioxide is used to a varying degree in the making of almost all wines, aside from being a bi-product of fermentation. Very few winemakers will make wine without the additional use of sulfites. Red wines however need the least as they naturally contain anti oxidants.
To put this into figures, EU Law states that the maximum permitted levels of sulphur dioxide are: 160mg/l for red wine; 210mg/l for white and rose; and 400mg/l for dessert wine.
Undoubtedly many quality driven producers, especially those working organically or biodynamically, will set their own strict limits.
When tasting wine the affect of sulfites will vary from person to person, with some being more sensitive than others. When noticeable sulphur dioxide has an unpleasant smell and resembles that of a struck match.
Finally, there are even some foods, such as dried apricots, that are naturally high in sulfites.