From Longman Dictionary of Contemporary Englishtell somebody/something apart phrasal verbRECOGNIZEif you can tell two people or things apart, you can see the difference between them, so that you do not confuse them SYN distinguish It’s almost impossible to tell the twins apart. → tell→ See Verb table
Examples from the Corpus
tell apart• Eventually, by the eighteenth century each strain had so much affected the other that it became impossible to tell them apart.• He looked at me and said he often wondered how I told my books apart.• It was hard to tell them apart.• Once you expose these to the New York atmosphere for three or four months, you can not tell them apart.• Salesmen, traders, and managers swarmed over the floor, and at first I could not tell them apart.• The governments and towns have got to find some way of telling them apart.• It would be impossible to tell the results apart but the methods used to achieve them are very different.• They can tell each other apart by their distinctive songs. 