A toothpick test is used to check the "doneness" of something you are baking, such as brownies or a cake. Because the outside of a cake cooks faster than the inside, a toothpick test helps make sure that it's done all the way through.

To perform the test:

  1. Locate a toothpick.
  2. Insert the toothpick into the center of the cake (or whatever) about halfway inside.
  3. Remove the toothpick immediately.
  4. Inspect the toothpick. Is there a bunch of delicious cake clinging to the end?
  5. If yes, then the cake is not quite done. Close the oven and set your timer for two or three more minutes. Repeat steps 1-4.
  6. If no (at least, no more than a few crumbs), then the cake is done. Remove it from the oven and enjoy.

Notes:
If you do not have a toothpick, feel free to use any similar wooden or ceramic bits you may have laying around the house, such as a kebab skewer, a chopstick, a piece of uncooked spaghetti noodle (thanks jessicaj!) or even a clean broomstraw. I have used a metal butterknife in the past, but if you have to use a metal utensil, you should hold it in your hand for a little while to warm it up first. Cool metal can cause a "done" cake to appear underdone.

The toothpick test is best used on cakes and similar baked goods, like muffins and brownies and bread.

Thanks to The Custodian for reminding me about the dangers of cold metal!
The Debutante says "Another very useful method to check for cakey-doneness is if it is 'singing'. Sponge cakes especially make a gently crackling/hissing noise when they're cooked." I had no idea!