tell (someone or something) apart
To be able to discern or distinguish two or more people or things that appear or seem similar. This one is real, and this one is the counterfeit—you can tell them apart by the small holographic mark at the top corner of the genuine document. Even our parents sometimes have trouble telling me and my brother apart.
tell people or things apart
to distinguish one from another. I can't tell Bob and Bill apart. I find it easy to tell apart Bill and Bob. The two cakes look different, but in taste, I can't tell this one and that one apart.