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 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.