Salifert makes an iodine test, not sure who else does.
I do not test for iodine, but I dose slightly less than the bottle instructions. I use Kent Lugol's solution, which is very concentrated -- 1 drop per 25 g of system volume once weekly. Our total system capacity is 125 g before any sand, lr, equipment, etc. I've estimated our total system volume to conservatively be 80 g, 85 g max, based upon how much RO/DI water I had to make to completely fill it up initially. That said, I dose 3 drops once weekly (as if our system volume was 75 g). I'm not afraid of having overly elevated iodine levels at that dosing rate as quickly as our Xenia grows and how often our shrimp, crabs, and amphipods molt. I certainly would not recommend dosing without a test to everyone else, it's just my choice, especially after reading the potential reasons for false results with the Salifert kit. It's a risk, be it very small if you know what you're doing and very large if you don't. If overdosed, the Kent Lugol's solution can be counteracted with Kent Chlorinex or Ammonia Detox at a rate of 2 drops per every 1 drop of suspected overdose.
EDIT: Seachem makes an iodine test kit, also.