A colloquial expression meaning 'which is which' — used when you cannot tell two things or people apart.
双子だから、どっちがどっちか分かりにくい。
They're twins, so it's hard to tell which is which.
資料の順番が入れ替わって、どっちがどっちか分からなくなった。
The documents got mixed up, and now I can't tell which is which.
目隠しされたら、どっちがどっちか全く分からないよ。
If I'm blindfolded, I can't tell which is which at all.