know which is which

know which is which

To be able to identify or distinguish two similar or identical people or things. Tom and his twin always dress so similarly that I never know which is which. I know that the two formats look very much alike, but you really ought to know which is which by this point.
See also: know
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

know which is which

 and tell which is which
to be able to distinguish one person or thing from another person or thing. I have an old one and a new one, but I don't know which is which. I know that Bill and Bob are twins, but I can't tell which is which.
See also: know
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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