If a lot are falling unripe it means the trees are a bit stressed. One reliable test is to juice an apple and use a hydrometer or refractometer to test the sg, if you can get 1.050 or 1.055 they are getting pretty ripe, though it depends on the season how high the sg will get. In a cool year you may not get over 1.045. Of course the easiest is to take a bite, you want them as sweet and full of flavour as possible. Don't pick them too soon, your cider won't have as much flavour, The main factor in the quality of cider is the ripeness of the apples