The sensations of pain vs ticklishness are different. While the bottoms of my feet are very sensitive both to pain and tickling stimuli, that isn't the case with everyone. As for tests you can use to determine just how ticklish someone is on their soles, instruments such as pinwheels, long fingernails, brushes, forks, toothbrushes, and even Q-tips can be devices of total torture for a woman with very ticklish feet (I used to get a former girlfriend howling with laughter using a small stiff paint brush on her toes, the balls of her feet, arches, etc; she was also very sensitive to a Q-tip being dragged around her toes then down the foot from big toe towards the arch (she would go out of her mind when I did that).