You're right, there is no Litmus test, they've even shown that different water sources can have an impact. In addition, if you use any moisturizing product on your feet, polishes, including pedicure products like acetone, that can all influence the minerals to change color.

The best detoxification advice I have encountered is plenty of drinking water or clear liquids, massage followed by a lot of liquids, and allowing yourself to perspire naturally by using only a deodorant as opposed to an antiperspirant.

P.S. You'll have to give up that nasty heroine habit, too.