We ran into some troubles because of a bone-headed screw up a few batches of beer ago and needed to find out if we had wrecked our beer with an inappropriate temperature too early in the process. If we had failed to exract fermentable sugars, or enough fermentable sugars we didn’t want to waste another 3+ hours monkeying with our brew. To find out whether we had screwed up our mash we performed an iodine test.
The procedure is simple. Get a small container and put a simple of your mash liquid in there (no grains!). Put a drop or two of Iodopher in there. If it turns black or blue you still have starches to convert. If there is no color reaction you’ve maximized the attenuation of your wort and you’re ready to sparge. It’s that easy and a best practice for good brewing.
Check out the homebrewing wiki’s page on the iodine test.