know somebody by sight

know (someone or something) by sight

To recognize someone or something based on its or their appearance, but without knowing its or their name or any other information. Everyone in town knows the iconic bridge by sight, but few know the history of when it was built and why it was designed to look so peculiar. Even after working there for a year, there were still some co-workers whom I only knew by sight.
See also: by, know, sight

know somebody by ˈsight

recognize 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.
See also: by, know, sight, somebody