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.
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.
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.