If you cannot or do not grow citrus, you can buy many varieties in the grocery store and at fruit and vegetable stands. Choose fruits that seem heavy for their size and have good, bright colored skins. There are some varieties that are ripe and delicious when still green. Valencia sometimes go back to green in warm weather but are still good. Avoid fruit with bruised or wrinkled skin.
Unfortunately you can not taste an orange before you buy it as you can a grape from the clump, but when you find a variety you have not tried before, buy a small amount and see how you like it.
Oranges vary in size and shape and some that look strange, like the Ponkans, are the most delicious. So look around and taste so that you don't miss that treasure. Do not be concerned with rusetting, a mottling of brown or tan on the skins. This often indicates thin skin and superior quality.
Oranges will keep well for a week at room temperature (best for juice) or at least a month, possibly as long as six weeks in the refrigerator or in a cool but not freezing place. This applies to most citrus.
Lemons should be smooth with small points on each end.
Limes can vary in color from bright green to pale yellow. Key Limes will be smaller and rounder. Limes will keep in the refrigerator for two weeks and at room temperature for one week.
Grapefruits should have a firm, smooth skin, the color of which will vary some depending on the color inside. The sweetest ones I've tasted had skin almost red. These will keep for a month in the refrigerator.
If you find a pummelo, buy and try it. It is delicious. You can eat it right away or keep it on a shelf for two weeks or more and it gets sweeter. These come in different sizes and shapes, mostly large, and some are bell shaped.
Tangerines come in many sizes and shapes and usually are a bit smaller than oranges and with naturally looser skin. They will keep in the refrigerator for several weeks.
If you live in a citrus growing climate and someone offers you fruit, take at least a few and try them. There are many delicious kinds that do not store or ship well and are never found otherwise.
from Citrus by Monica Moran Brandies