How can I be sure the whole potatoes I buy are of superior quality?

Most conventional potatoes are now marketed in 3-, 5-, or 10-lb. see-through bags. The potatoes should be firm rather than rubbery and free of cuts and bruises. Avoid potatoes that have a greenish hue, as they have been exposed to excessive sunlight and will taste bitter. Green potato skins are toxic. Also, avoid potatoes with sprouts, which denote the potato's starch has begun to convert to sugar. The potatoes will have an unwanted sweet flavor.
 
Be sure the potatoes aren't moldy. Of particular concern is a parasitic fungus that can attack potatoes while they are growing. Infected potatoes develop grey or dark patches that appear reddish brown beneath the skin,. They quickly decay to a foul-smelling mush due to a secondary attack from bacteria.
 
When selecting sweet potatoes, choose firm, dark, smooth specimens without wrinkles, bruises, sprouts, or decay. Even if cut away, a decayed spot may cause the entire potato to have an unpleasant flavor.
 
Source(s):
psgrill.net "Potatoes"
Centers for Disease Control "Vegetable of the Month: Sweet Potatoes"
Organicfacts.net "Potato Sprouts--Are They Good?"
Wikipedia.com "Phytophthora infestans"