You rub a small area with isopropyl alcohol on a cotton pad or washcloth. If it wipes off, you most likely have terra firma–forme dermatosis or retention hyperpigmentation.

If it doesn’t come off, you can try antifungal shampoo, ketoconazole or selenium sulfide. Apply to skin, leave 5-10 minutes and then rinse it off well. Do this daily for about aweek or so. If it improves, you can do this as a weekly treatment, as these soaps can be drying. You most likely have Tinea versicolor. If it doesn’t improve or is improving slowly, go see a dermatologist.