Buy a 500ml bottle of Betadine from the chemist. Make sure you get the brown bottle, not the blue (which has a lower concentration of iodine).
Put about a teaspoon of grains from the mash on a plain plate, drop on about 2-3 drops of iodine, wait 10-20 seconds, the iodine goes black if there is still starch present. If all conversion has taken place, the iodine stays brown.
Just so you know what you are looking for, try an iodine test at the very start of the mash. Lots of starch, lots of black.
If you leave a fully converted sample a while, you can see traces of black directly under the husk, as the iodine seeps in and reaches an unmashed section of the grain.
I did an iodine test for a few mashes, they all were converted, haven't done a test for the last 50 or so brews.
The iodine is also used for sanitising, 2.5ml per litre, contact time 10 minutes. Drain, then use equipment. 500ml will last a while.