well its hard to tell.
when cactus dries sometimes its dark green other times its really light.
mold is easily identified though, its mushy, it smells disgusting like mold, its fuzzy, and it deteriorates the material that it is on. a dried piece of cactus is going to be hard. therefore, if your pieces are hard and dark, you are fine, but if theyre still mushy, dark, decomposing, fuzzy, and releasing spores, well, needless to say, thats bad.