which is which
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.

[TahlilGaran] Advanced Learner's Dictionary ▲

which is which
n. phr. Which is one person or thing and which is the other; one from another; what the difference is between different ones; what the name of each one is.
Joe's coat and mine are so nearly alike that I can't tell which is which.
Mr. Hadley hadn't seen his friend's daughters in such a long time that he couldn't remember which was which.
Compare: WHAT'S WHAT, WHO'S WHO.

[TahlilGaran] English Idioms Dictionary ▲