can't tell apart; indistinguishable
Used when two or more things look or seem so similar that one cannot distinguish between them. Often implies a lack of clear difference.
See also: 区別がつく
I really can't tell these twins apart.
It's so elaborate that you can't tell the real thing from the fake.
Similar meaning but emphasizes visual distinction; 区別がつかない can be used for abstract differences as well.
Standard kanji spelling using 区別.
Alternate spelling using 付く for the verb; both are used.
Phrase composed of 区別 (distinction) + が (subject marker) + つかない (negative potential form of 付く, 'to be attached/perceived'). Literally 'a distinction does not attach', i.e., one cannot perceive a difference.