Give the fruit a squeeze, if it has some give the fruit is ripe or close. To double check give the fruit a slight twist. If it separates from the limb easily it is good to go. Apples, pears, plums, peaches, apricot will all twist off easily when ripe. Usually the birds will also be an indication that fruit is ripe. The birds will be start eating the fruit as soon as it becomes ripe. I also think the birds deserve some fruits of their labor, so I like to leave a few as a bit of an offering.
