They look exactly the same, so I can't tell them apart.
I can't tell the difference between them because they look so much alike.
ーThey look exactly the same, so I can't tell them apart.
「彼らは全く同じなので識別できない」
to look exactly the same で「全く同じように見える」
can't tell ... apart で「識別できない・見分けがつかない」
ーI can't tell the difference between them because they look so much alike.
「彼らはすごく似ているので違いがわからない」
can't tell the difference で「違いがわからない」
to look so much alike で「とても似ている」
ご参考まで!