You use which in questions when there are two or more possible answers or alternatives.

You use which to refer to a choice between two or more possible answers or alternatives.

You use which at the beginning of a relative clause when specifying the thing that you are talking about or when giving more information about it.

You use which to refer back to an idea or situation expressed in a previous sentence or sentences, especially when you want to give your opinion about it.

If you cannot tell the difference between two things, you can say that you do not know which is which .