I am gardening for the first time this year (yay!) and have several healthy tomato plants that set heavy crops (yay!). I’m a sucker for lots of color, so I planted red, yellow, pink, and green varieties. Right now, some of the yellow and red varieties are starting to ripen (yay!)
But… um… something occurred to me. I don’t know how to tell when the inherently green varieties are ripe.
Help?
Watch for them to stop growing bigger. Then you had better pick some and try them. Tomato seeds will sprout in a tomato if left for a long time.
We have heirloom tomatoes and we pick them when they are about the size of a softball. If some are still green, simply put them in a Paper bag for a day or two to ripen.
Read carefully, darling. They’re MEANT to be green 
. This is why I can’t find the answer on Google… all the hits are about ripening “green” tomatoes that eventually turn other colors. These are meant to be green, even when they’re ripe and sweet.
I’ve never grown heirloom green tomatoes, but a regular red tomato, when it’s ripe, not only is red, but it’s also soft–it “gives”, like a ripe peach–and fragrant, especially when sun-warmed. If it was me, I’d see if they were soft, and smelled like ripe 'maters, to give me a hint. If nothing else, bite into a couple; that’ll give you an experience benchmark. “Well, when they looked like this, I tried one, and it wasn’t ripe…”
We grow a green zebra variety because you can tell they’re ripe when the stripes start to turn yellow. No idea how to tell the ripeness of a solid green tomato, sorry!
My office manager grows many varieties of heirloom tomatoes. Her answer: when they are ripe :D. She picks and tastes. Along with the green ones, she has brought in purple tomatoes, and one variety that tastes like a peach (a little bit).
I have grown green heirlooms, and this is good advice. Also, as mle points out, some green tomatoes have some contrast to look for.
I grew ‘green grape’ tomatoes a few years back - when they’re unripe, they look a sort of dull, pale, flat, solid green. When they ripen, they turn a bit translucent, marbled and deeper, clearer green. They’ll also be softer.