I'm slowly converting over to all grain and have only a limited experience with stove top partials and a single all grain recipe. So, maybe I'm way off base here, but I suppose you could calculate what the gravity of the wort in the tun would be after conversion, draw off a sample and measure it, but iodine's probably just as easy. Of course, I just make sure the temperature and grist to water ratio are correct and let it sit for an hour and don't do either. In theory, if the conditions are correct the mash could fully convert in 15 or 20 minutes, but leaving it an hour insures that everything has plenty of time to convert.