How one can know if a green-apple is ripen?

If the tree is supposed to produce red apples, and an apple hasn't ripened to red during the time expected, you might want to just pluck it and discard it.

Sorry, but I don't think you've answered to me. Have you really red my question, please? I'm talking about green apples which NEVER will be red. That is why they are called green apples. It seems to me you live a country where green apples are even more rare, in such an extent that you've never heard about them.

Here’s a thought: why not taste it? If you have that type of apple, then eat it. If you haven’t got that sort of tree, then it’s not an issue, right? What am I missing?

smell helps. we used to pick winesap apples by cutting one and smelling it. It has a real apply smell.

Yes, I read your question.

You never actually said you were asking about a tree which grows green apples ("now we still have on" - this is a statement which doesn't scan in English). Those are called Granny Smith, by the way.

Then farmerman's advice stands. Cut one down and taste it.

Bramleys are green too. You don’t want to eat one of them raw.