The best way to check Iodine is to paint the bottom of your feet with Lugol's 5% at bed time. 4 drops on the smooth areas of each foot, spread out with a glass rod and let dry.
In the morning, when you first wake up, check each foot and see if it is still stained. If so, then you are getting sufficient Iodine. If both feet are clear, then you need more.
Repeat each night until the feet are still stained when you wake. When this happens, decrease Iodine to 3 drops, if still stained after doing this a couple of days, decrease to 2 drops, etc.