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

它们在我看来简直一模一样，我永远分不清哪个是哪个。

《柯林斯英汉双解大词典》

Now which is which?

现在，哪个是哪个呢？

youdao

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。

《牛津词典》

Which is which? Both,I guess.

VOA: special.2009.09.20

So each of these cups again have different weights if I want to find the lightest of them I might start not knowing which is which.

它们的重量互不相同，如果我要,找出最轻的一个，而我根本不知道会是,哪一个。

哈佛公开课 - 计算机科学课程节选

which is a city in South Eastern Africa, which is a beautiful, beautiful city which everyone should go to.

非洲东南部的一个城市，很美很美，人人都应该去。

世界是我的家 - SpeakingMax英语口语达人

Obviously there is a correlation, but which is the cause and which is the result?

FORBES: Innovation and Growth: Which Comes First, the Chicken or the Egg?

Best to let the social scientists figure out which is cause and which is effect.

FORBES: Magazine Article

You make news of something which is not news, which is not a thing in Netherlands.

FORBES: The Unbearable Whiteness of Piet-ing: Dutch Santa's Blackface Elves