問人想搵邊個...

Can I help you?

When you see a stranger walking into your office, you'll say "Can I help you?"
If he is looking for somebody, he will tell you. You don't need to ask that offensive question.

When you are on the phone, you ask "May I know who is speaking?"

Native English speaker does not say " Who are you looking for? 搵邊個" unless the speaker does not welcome the stranger.

可以用 who are u looking for ?