Hi Gary,
My Custard Apples /Annona Reticulatas get ripe right on the Trees, the skin gets shiny and u can smell them when there's several ripening at the same time, the first one's to smell them are the critters, I have gotten used to watching them and just picking them when I think the color is at it's best. My Trees are about 20-25ft tall so squeezing them is out of the question.
If you can squeeze them, they should be soft, too soft and they drop.
Hope it helps.
Carlos