At the entry point, just ask "Hello, is this Business Class Check-in?" (Make sure you don't have preprinted tickets out.)

Almost everywhere is self check-in so by the time you see speak to the next person you'll have your tickets and your bag tags will be printed.

If for some reason you get really "lucky" and a person checks you in, ask them to let you since this is a United-issued international business-class ticket.

As cfischer says, hard to imagine them turning you away.