We've started to take BRIX (percentage of sugar to juice) readings in our vineyards. I like to harvest grapes at between 23.5 and 25.0 BRIX. It depends on the vineyard and other factors exactly when we pick, but those are the 'numbers' I look for.
We also look for 'secondary' signs of ripeness. The pips or seeds should become brown and crisp. The petiole, where the cluster connects to the vine, should harden and turn brown. The skin should start to dimple on the grapes, and when you eat a grape the flesh should tear with your teeth and the pulp should separate easily from the seeds.
I also check for flavor development. Grapes change flavors as they get ripe. Syrah starts of very citrus like, with pineapple, and tropical fruit. It then gets very peppery with raspberry and cherry fruit. Next it starts to develop deep cherry and berry fruit, the pepper gets more like black pepper and finally you get fruit flavors like wild berries and dark plums. If you leave it on the vine long enough, you'll get prunes, dates and raisin flavors.
I like to pick when the flavors are in the middle area, with still a hint of red fruit, but moving into the dark fruit flavors and before the prune flavors show up. In our vineyards that usually happens right around 24-25 BRIX.
Right now most of the vineyards are right below 20 BRIX in the 'warmer' vineyards, and 18 in the cooler ones. That means we are still 3-4 weeks away in the warmer ones and 6-8 in the cooler ones. Secondary signs are just starting to show, and flavors actually seem a little ahead of the BRIX. We'll probably pick the grapes around our home, the Haut Tubee, in about two weeks. That is always our first pick of the season.