expression
which is which; one or the other
Used when the speaker cannot tell two things apart, or when the distinction between two options is unclear. Often appears in questions or statements of confusion.
この二つのボタン、どっちがどっちかわからなくなった。
I can't tell which of these two buttons is which anymore.
双子なので、どっちがどっちか見分けがつかない。
They're twins, so I can't tell which is which.