You want to pick the fruit with the biggest carpels to get fewer seeds and more pulp. Large carpels also indicate that the fruit was left on the tree long enough to mature. Another great judge of maturity is whether the seeds rattle or not. If the seeds don't rattle, but everything else checks out fine, then get the fruit anyway. Rattling seeds are just a shortcut to looking at all of the other characteristics.
Color is another thing to look for.
As cherimoyas mature they turn from a medium green to light green or even yellowish green. If the cherimoya is turning yellow then you can be sure that it is mature and probably super sweet, but green cherimoyas can be very good too. In cold weather the skin doesn't change color as quickly as the insides mature, so don't ignore one just because it's green. If you have to choose between two otherwise identical cherimoyas, choose the larger one. In my experience, the larger one will be sweeter.