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.
tell things apart
to distinguish one thing or a group of things from another thing or group of things. This one is gold, and the others are brass. Can you tell them apart? Without their labels, I can't tell them apart.
tell apart
Discern or distinguish, as in It's hard to tell the twins apart. [First half of 1900s]
tell apart
v. To perceive something as being different or distinct from something else: I couldn't tell apart the real $20 bill from the counterfeit one. The twins were identical, and we couldn't tell them apart.