Can a cavity and simple staining be told apart?

Yes, an examination can tell them apart. Simple staining is pigment from coffee, tea, or smoking sitting on the tooth surface; it comes off with scaling or polishing and the tooth itself is healthy. A cavity, by contrast, is a lesion where the tooth has actually decayed, so it does not wipe away, the surface feels soft and hollowed when probed, and it shows as a shadow on an X-ray. Even when the dark spot looks similar, 1) a brown line following the grooves of the chewing surface is often staining, while 2) a hole you can feel, or accompanying sensitivity, points to a cavity. It is safest to have it confirmed with a dental examination.