A discussion on Chowhound recently pondered the question: What are the signs of good prosciutto?
First, real prosciutto is dry—beware the moist slices resembling lunch meat that you find in plastic packages at the supermarket, splatgirl says. Good prosciutto is dry-aged, resulting in a firm, dry product. Sliced thin, it should be silky, pliable, and soft, not leathery, mitch cumstein says, and the fat should be a brilliant white. Also, it should taste intensely, viscerally porky: "[a]n umami bomb," splatgirl says.
Almost any prosciutto imported from Italy is likely to be decent, splatgirl says, but "[a]void like the plague anything from any of the mass-market continental 'deli' brands even if they try to trick you with a faux Italian name." The only domestic prosciutto worth buying, in splatgirl's opinion, is La Quercia. It's sold to stores in small packages, so it's less likely to go stale in the deli case.
Photo of whole prosciutto in the deli case at Salumeria Rosi, Manhattan, by TrishUntrapped