Simply tasting a grape won’t give you an exact determination of its readiness, but you’ll definitely know if it’s too tart. Another, more scientifically sound way to check sugar content is to use a refractometer.
A refractometer allows to you see the amount of sugar within a grape. You use it by squeezing a sample amount of the juice from a grape onto its receptacle and looking through to see what levels the sugar is at. In the image to the left it is at a low number of 15, you usually want to wait to pick your grapes until they are in the 22 range. Many factors can affect how long it takes before grapes are harvest-ready, so it’s always important to work on their schedule.