know somebody by sight
know (someone or something) by sight
To recognize someone or something based solely on appearance (without knowing any other information, such as a name, etc.). Even after working there for a year, there were still some co-workers I only knew by sight. I'd know the place by sight, but I don't think I could tell you how to get there.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
know somebody by ˈsightrecognize somebody and know who they are, without having spoken to them: I haven’t actually met Dr. Galston, but I know him by sight of course.
Farlex Partner Idioms Dictionary © Farlex 2017