Which is which
Definition 1 of 1
Pro Tip 1/3Clarifying Doubts
Use 'which is which' when you want to ask someone to clarify the identity of similar items or people. 
The twins are dressed alike; can you tell me which is which?
Pro Tip 2/3Similar Choices
'Which is which' is often used when you're presented with options that look alike but may have subtle differences. 
These two colognes smell almost the same; which is which?
Pro Tip 3/3Figuring Things Out
The phrase implies that the speaker is trying to solve a confusion or mystery about similar things. 
Both mushrooms look edible, but we need to figure out which is which before using any.