Deciding which orange (Citrus sinensis), growing in U.S. Department of Agriculture plant hardiness zones 8 through 10, will be the sweetest can be a challenge. If you picked the orange off the tree yourself, you know the color is accurate, as opposed to the oranges found in stores that may have dyes added to enhance the orange color. But the bright orange coloring is not the best indicator of whether or not an orange will taste sweet. Several other factors such as weight, type and scent must also be taken into consideration.
Take note of the type of orange you are examining. Valencia oranges (Citrus sinensis), growing in USDA zones 9 through 10, are available in the spring through fall and have a very sweet taste. Cara cara oranges (Citrus sinensis "Cara Cara"), growing in USDA zones 9 through 10 and available the first few months of the year, are sweet, but with a cranberry-sour tang that Valencias don't have.
Choose an orange with no shriveled or moldy spots. Brown spots are formed in hot climates or from rubbing against the tree and don't affect taste. Valencia oranges may have green spots, but these spots mean the orange ripened on the tree, then started to re-absorb chlorophyll. It does not mean the orange is not ripe.
Pick up several oranges of similar size, two at a time, for comparison. The orange that is heavier for its size will be juicier and sweeter. The thinner-skinned of the two oranges is also likely to be sweeter.
Scratch a small area on the skin of the orange. Sniff the exposed spot. You should be able to smell the sweet orange scent, letting you know the orange is sweet.
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