The most reliable way is to use the toothpick test. Insert a toothpick or skewer into the centre of the cake. If it comes out clean or with a few moist crumbs clinging to it, your apple loaf cake is done. If it comes out with wet batter, give it a few more minutes and test again. Remember, every oven is slightly different, so it's always better to start checking a little earlier than the suggested baking time.