どっちとも言えない
I can't tell which is which.

A: Is this a real diamond or a fake?
B: I can't tell which is which.
A: これは本物のダイヤモンド、それとも偽物？
B: どっちとも言えないなあ。

★I can't tell which is mine.なら「どちらが自分のものか分からない」。