Citrus ripens in an unusual way. With little visible change to the outside peel, the fruit inside changes from dry and sour to juicy and sweet over a period of weeks or months.
Malcolm Manners recommends testing one or two for flavor before you pick a lot off your tree. "Palates vary, so your citrus is ripe exactly when it tastes ripe to you."
Also, he explains, young trees--especially grapefruit and navel oranges--may bear only dry, pithy fruit for the first year or two. "After the third or fourth year, most citrus trees can be expected to bear generous quantities of good fruit, and they'll keep producing at this level for 25 years or more."
Tip: Remove any fruit from a newly planted tree so it will put all its energy into growing. You'll raise a better, stronger tree by sacrificing this first little crop.