which is which idiom
哪个是哪个（用于区分两个相似的人或物时）
used in expressions that relate to being able to see the difference between two very similar things or people
For the first few months the babies looked so alike I couldn't tell which was which. 头几个月里婴儿们长得都很像，以至于我分不清哪个是哪个。