I never trusted that whole "Put in a toothpick and if it comes out clean, it's done" thing. If there's still batter on that bad boy, you're waaaaaay premature (even when not completely baked, it can still come out clean) and may want to ignore that cliched tip. Stick with the directions in your recipe and it should at least get you close to a point where you achieve desired results but always keep an eye on it on a first twirl. If it's way off it may be time for a new oven. Meat thermometers are good to determine "doneness" of meat, chicken and fish but that seems overly anal to me.
Long and short of the tip: Start like 2 minutes below the recommended cook/bake time of your recipe and then monitor.