Know someone by sight:
If you know someone by sight, that person looks familiar to you, but is not a friend of yours.
如果你 “know someone by sight”，意思是你 “看某人面熟，但可能对这个人还不够了解，不知其名或没说过话”。
例句
Do you see that person there? I know her by sight, but I can’t remember where I met her.
你看到那个人了吗？我看她面熟，但就是想不起在哪里见过她。
When I went to the industry conference, I noticed a lot of people by sight. I must have met them at the same event last year.
我去参加行业大会时，很多人看着都眼熟。我一定是去年参加这个活动时见过他们。
I know him by sight, but I don’t know his name.
我只是看他面熟，但我不知道他叫什么。
