How can I be sure the cherries I select are fresh and wholesome?

Look for plump, firm fruits with their stems intact. Also look for cherries that have been kept cool and moist, as flavor and texture both suffer at warm temperatures. Cherries should be large--an inch or more in diameter-- and glossy and plump. The cherries may be light or dark red, though the color of ripe cherries may vary, depending on the variety. For instance, bing cherries are deep red, Raniers and Royal Anns are golden pink, and black cherries are mahogany-colored.  Two sour varieties, Montmorency and Morello, are bright red.
 
Stems should be fresh and green. Avoid cherries without stems because the resulting skin break presents an opportunity for decay to commence. Check carefully for bruises or cuts on the dark surface. Avoid cherries that are sticky from juice leakage. In general, a ripe cherry is heavy for its size, which indicates that it has plenty of juice and is meatier and sweeter than an immature cherry. Like melons, cherries don't continue to ripen once they are picked. Avoid cherries that are hard, small, and/or lighter in color because they were probably picked before they were ripe.
 
Source(s):
Whole Foods "Guide to Fruits"

Shine.com "How to Pick the Best Cherries"
Tonytantillo.com "Fruits: Cherries"