Sugared plums sometimes conjure up images of sweet desserts and Christmas holidays of old, but the phrase "sugared plum" cannot be applied to a fruit that is unaltered by cooking or coating with sugar. Unless you are preparing a candy recipe of the same name, the only signs of sugared plums you would encounter are dried plums---or prunes.
While there are about 15 recognized species of plums, the Oxford Encyclopedia of Food and Drink in America notes that European plums contain enough sugar to dry them whole, without fermentation. The U.S. imported a variety of European plum called Agen from France in the mid-19th century. Agen plums have been grown and cultivated in the California Santa Clara and Sacramento Valleys. Dried Agen plums were used frequently in American kitchens until the mid-20th century, when cold storage and fruit imports became more common. Today, European plums are usually sold dried and are likely the closest you will come to seeing a naturally occurring "sugared plum."
According to historicalfoods.com, creating real sugared plums takes almost three days of alternating oven-drying with rolling in sugar. Many sugared plum recipes date back to the Victorian Era when sugar-coated fruits, nuts and seeds were made to hang on the Christmas tree. But as Ivan Day's historicfood.com tells us, "Every Victorian child knew that 'sugared plums' were not sugar plums."
"Sugar plums" were a form of boiled sweet, says An A-Z of Food and Drink. The candied creations had the same shape and size as plums but were often made with other foods, starting out as caraway seeds or almonds, for instance. Using syrup and numerous drying periods in between, the comfits grew in size as the coatings were built up in the sweet-making process, says historicfood.com.
While the Postharvest Technology Research and Information Center of the University of California-Davis describes certain signs of ripeness, bruising, rot or freezing damage on plums, there are no signs that will predict a plum's sugar content.