Yes, pretty much what TZ already said, look for the chartreuse
(yellow/green) color. This was my first year growing Green Zebras
too. As you continue to watch them, the color change is obvious when it starts to happen. Also, the tomatoes do feel a little softer as they ripen.
On my tomato sandwiches, I use a good "red" tomato, and also add a layer of Green Zebra
... heavenly !