This is Italia, a seeded grape that can be used for wine or fresh eating. Note the berry color change from a green (not pictured) to a yellow green indicating time is near to harvest. Pick Grapes. The easiest way to know if grapes are ready to pick is to taste them and look at them. If they are sweet, go ahead and cut off a bunch. Dark colored grapes are easy to tell if they are close to being ready. Green grapes are more difficult but there is usually a slight color change from green to more of a yellow green color.           Grape bunches on the same vine are not all ready at the same time. You should be able to selectively harvest grape bunches over a period of about two weeks if you continue to irrigate the vines. Withholding water from a vine helps to push the harvest days closer together. You begin to withhold water when you see the color change in the berries. Handheld refractometer for measuring sugar content of fruits and vegetables           In this heat and intense sunlight, grapes will add about 1% sugar content every 2 to 3 days. With table grapes we will harvest them at around 20 Brix. With wine grapes on the other hand we like to see at least 24 Brix. One Brix is equivalent to about 1% sugar by weight. The higher the Brix reading, the higher the sugar content.           But most people are pretty accurate with sweetness just by taste. The device which gives sweetness A numerical value is called the refractometer. Refractometers can be purchased at many places on the Internet.