1 general:: Phrase(s): know someone by sight to recognize a person’s face, but not know the name. • I’m afraid I don’t know her by sight. • I know all my employees by sight.
McGrawhill's American Idioms And Phrasal Verbs
2 general:: Recognize someone or something by appearance but not know the name or other details. For example, I know a lot of people by sight from the tennis courts. [1200s]
American Heritage Idioms