Nectarines are best during high summer months. Look for red skin with golden undertones for yellow nectarine. They will have a sweet aroma as well when ripe.  Nectarines that still have greenish hues on the skin, whether they be white or yellow nectarines, were picked too early. You can also check ripeness by giving the nectarine a gentle push. The softer it is, the more ripe the nectarine.