That's a good question and something I was thinking about myself. My worry is more that they're claiming it's 5% Iodine but it's really diluted with something cheap like alcohol. For instance, you read about GNC and Walmart brands got caught diluting herbal supplements like st. johns wort.
I thought about the starch test. You know from junior high class where you use Iodine to test for starch and a potato will turn dark blue?
This is my plan:
1. Get a bottle of 5% you know or feel confident is truly 5% as the control sample.
2. Corn Starch
3. Distilled water
4. Two small glasses (maybe shot glasses?)
Take two small glasses of water with exact same amount of distilled water. Put exact same amount of corn starch in each and mix the starch water.
Then add a drop of 5% of your trusted source into one of the two and note the color. It should turn dark purple or bluish/black, depending on the amount of starch you mixed in the glass.
Next add a drop of the new bottle you're not sure about into the second glass. Then see if the color matches.
I'm still thinking about how to do this experiment right. Maybe it won't work since I haven't tried it yet and probably haven't thought of some angles. So, if someone has some thoughts please chime in.