know by (something)
1. To be able to understand or recognize that something is the case because of something. In this usage, a relative clause can be used between "know" and "by." I knew by the look in her eyes that she wasn't telling the truth. You'll know when the chemical reaction is complete by the sudden change in the solution's color.
2. To be able to identify, distinguish, or recognize someone or something by a particular trait or characteristic. In this usage, a noun or pronoun is used between "know" and "by." We'll send Mike to come meet you. You'll know him by the bright red jacket he always wears. I've always known this building by the iconic statue in front of its doors. I only know him by name. I've never met him before.
3. To be able to recite or recount the entirety of something from memory alone. In this usage, a noun or pronoun is used between "know" and "by." Typically followed by "heart," "memory," or "rote." OK, everyone, please keep practicing. I want you all to know this song by memory on Monday, all right? I've read this book so many times that I know it by heart at this point. Knowing the solutions by rote is not as useful as being able to work out the problems in real time.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
know someone or something by something
to recognize someone or something by a certain characteristic. I know her by her perfume. I know this committee only by its reputation, which is not good, by the way.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.