Bad goji berries are dark red in color

"The farmers must be very careful not to touch the fruit with their hands during the drying process because this will cause the fruit to turn very dark, which is unacceptable."

" At the herb shop, you may select your Lycium by looking at it, feeling it and tasting it. It should have a uniform color that is not too bright. Some suppliers in Asia dye the fruit red to make it more "attractive" to naive buyers. It should not have any dark fruit mixed in---dark fruit is oxidized and spoiled. The fruit should be firm and not mushy. Mushy fruit is spoiled. Larger fruit is best. Always taste Lycium before you buy it. It should be delectable. Great Lycium is very sweet and a total pleasure to eat raw or cooked."

Source :
http://www.qualitychineseherbs.com/herbal_ingredients/lycium_fruit.htm

Buy from reputable suppliers. Note that chinese herbs comes in a variety of grading. The price, visual quality and taste is your guide. Cheap goji berries are typically dyed red (bright, super-red). Those are meant for the typical consumer who use them for food (soup).

http://images.google.com./images?um=1&hl=en&q=lycium+gou+qi+zi&btnG=Search+Images