When the segments get flatter and the fruit gets a lighter color. Ripening is rather fast on sugar apples, it can ripen overnight. It may be rock hard today but can get very soft tomorrow. So, when it gets bigger, you start a daily check by giving it a little squeeze. Once you feel a little "give" you can pick it and let it ripen on the kitchen table, if you miss that stage, the ants and the flies will get it before you do. That fruit can still ripen but it isn't ready yet.