One quick way to tell is to check the blossom end scar. When you see a pattern in the skin and flesh color beneath it around the blossom scar that looks like a star, chances are it's close to ready! Also, you'll also notice the fruit will become a bit softer to the touch (duh), and you should still notice a bit of color change when compared to other fruits on the vine.
I'm having similar problems trying to tell when my Aunt Ruby's German Greens are ripe. Luckily for me, the Aunt Ruby's have the slightest hint of pink in a few key spots when they're ready to pick, but remain green inside.
Good luck with this Chris, and hope this helps. I've not grown this particular variety of green, but I'm betting the blossom scar thing should help a little. Maybe our tomato goddess will chime in on this one.