In my quest for a perfect shakeshouka (eggs in poultry) I figured out I need less acidity in the dish, and have read that acidity in tomatoes varies widely. I have read that very ripe tomatoes have lower acidity, but how do you know it is ripe? Most are picked before ripening before they are shipped. Does the ripening that follows after shipping matter in acidity?
Smell and touch your tomato. The softer it is, the riper. Also, the more it smells like a sun-ripened tomato, the riper. The smell is not only stronger, it is different from the smell of a badly ripened (but still red) tomato, or worse, previously refrigerated tomato. If you don't know how a good tomato smells, you'll have to learn it by smelling tomatoes before cutting them and remembering how they smell compared to their taste. Or go to somebody with a tomato garden in a reasonably sunny location, pick the ripest tomato and smell it before eating. I doubt that anybody in the world has developed the vocabulary to simply describe what you're looking for in the taste, so you have to experience it for yourself.
Don't go by color, that depends more on variety and can also be created with artificial ripening without imparting sweetness.