I know many chefs and cooks who keep a cake-tester tucked into pen-pocket on their whites and none who use them to test cakes. Not that you can't test a cake's doneness with them, it's just why would you want to when there are so many more interesting assisted-poking tasks at which it excels?
Essentially a heavy-gauge wire with a handle, it's about as simple as tools can get. The idea is that you poke it into the center of a cake and pull it out. If it comes out clean, the cake is done, sort of like a glorified toothpick. But the fact that it's long and made of metal means that it's useful for all kinds of other things.