Great question. Just like with brownies, our standard methods aren't very useful. If you stick a toothpick into a perfectly done blondie, chances are you'll come out with a little goo. With blondies there's a little guesswork involved, but better to under-bake than to over-bake. My advice is to stick with the recipe directions, but definitely check that your oven temperature is accurate with an inexpensive oven thermometer. It's worth its weight in gold.