Green When Ripes or GWR tomatoes are a very interesting category. They look like they are never going to ripen and it can be difficult to tell when they are ripe.
One way to tell is to watch for an amber or pink blush to develop, starting on the blossom end or bottom. Another good way is to give them a gentle squeeze. They will feel less firm with a little give to them.
Flavor tends to be sweet and slightly spicy and is a favorite of many. This category is growing rapidly in popularity.