Before brewing, go to your local drug store and pick up a bottle of Tincture of Iodine. That's the stuff often placed on cuts as an antiseptic.
When you think your mash may be finished, extract a small sample from it. Add a drop of iodine to the sample. If you see the sample turn dark purple or black, there is still starch in the mash to be converted. If there is little to no color change, your starches should be fully converted to sugars. Time to move on to the boil!
Below is a picture of what the test results look like. The tube on the left contains starch and has turned a dark color. The one on the right does not.
|Image courtesy of imgarcade.com|