I hold my thumb against the knuckle where the tomato stem meets the truss stem, and then gently lift the tomato against it. If the fruit separates easily at the knuckle then it’s good to harvest, otherwise I tend to leave it a while longer. I tend to pick little and often, then keep the fruit on a tray in the house until I am ready to use it.
Top tip, never keep in the fridge, you lose the flavour.
Second top tip, any unripe fruit left that you have to save from the frost, keep it in a bowl with a ripe apple or banana to ripen up. They give off ethylene gas which ripens them quickly - that’s what they do in commercial production, though I don’t suppose they use ripe apples!
And anything left over green in the end can still be added to soups, stews etc, just not quite the same flavour.
Happy harvesting everyone!