There is no good single way to test for fruit ripeness.
Brix - can be measured with a refractometer like the Atago you're looking at.
Starch - iodine can be used to determine the degree of starch conversion to sugars.
Seed color - the darker the seeds, the "riper" the fruit.
Acidity - acidity drops as fruit gets riper, so if you measure and can follow through the ripenign period, then you can get an idea of when the fruit getting towards full ripeness.
Ethylene - if you are lucky to have access to a gas chromatograph, you can take an air sample of the fruit core with a syringe, inject it in the GC and measure the amount of ethylene in the fruit. And like acidity, you can follow this through the ripening period. The higher the ethylene, the riper the fruit - simply stated.
Fruit color - sometimes darker, redder fruit is considered to be riper.
In reality, these used together are all good ways to determine ripeness. From a cider perspective, the use of brix, starch, and acidity are the best way to determine when fruit is ready/ripe. However, the best way is to taste the fruit!
Mike Biltonen, Know Your RootsZone 5b in New York