You can tell when blondies are done because they'll start to pull away from the sides of the pan and the edges will be light brown. Be sure not to overcook your blondies or they might turn out overly hard and dry. You can use a toothpick into the middle of your pan to test for doneness. However, a probe thermometer is the best way to check for accurate doneness. An internal temperature of 200-205℉ will give you a moist but fully cooked blondie.