If you know someone by sight, that person looks familiar to you, but is not a friend of yours.

Mike spends a lot of time in this area, so you'd probably know him by sight.

I knew Jo's husband by sight, but I'd never really met him.

He walked up to Rose, whom he knew by sight.

The guards wouldn't let him enter because they didn't know him by sight.

I'll introduce myself to you father - I'm sure I'll know him by sight.

See also: know verb (BE FAMILIAR WITH)