which is which是一个习语，意思是：

（区分人或事物）谁是谁，哪个是哪个used to talk about distinguishing one person or thing from another

The twins are so alike I can't tell which is which.

这一对双胞胎长得一模一样，我分不清谁是谁。

They all look so alike to me that I'm never sure which is which...

对我来说他们看起来全都非常相似，我永远搞不清哪个是哪个。

柯林斯高阶英汉双解学习词典》

It's essential to know which is which as treatments will be quite different.

由于治疗的方法将会全然不同，因此弄清楚哪个是哪个很重要。

《柯林斯高阶英汉双解学习词典》

which is which是何意