In what way is the existence of the Oompa-Loompas discovered by the members of the group touring the factory?

Upon touring Mr. Wonka's factory, Veruca looks up and sees a group of five little men across from the touring group on the other side of the chocolate river. Shrieking in surprise, the members of the tour group ask Mr. Wonka many questions about the Oompa-Loompas.