which is which

which is which

Which of these two similar or identical people or things is distinct or separate from the other in some specific way. Shoot, we both brought the same kind of backpack to class. Do you know which is which? The two political parties have become so similar over the years that I honestly have a hard time telling which is which.
Farlex Dictionary of Idioms.

which is which

What particular one is what particular one, or what is the difference between different ones. For example, These twins look so much alike I can't tell which is which, or Both our raincoats are tan; do you know which is which? This idiom was first recorded about 1412.
The American Heritage® Dictionary of Idioms by Christine Ammer.

ˌwhich is ˈwhich

used to talk about distinguishing one person or thing from another: The twins are so alike I can’t tell which is which.
Farlex Partner Idioms Dictionary
See also:
  • know which is which
  • tell which is which
  • shoot-out
  • go (one's) (own) way
  • go your own way
  • discriminate between
  • discriminate between (someone or something)
  • discern
  • discern (someone or something) from (someone or something)
  • discern from