How do they tell if you visited the control?

Mounted or hung near each control flag is a punch. An orienteering punch is a bright red plastic device with a number of sharp metal teeth. The orienteer uses this punch to impress the pattern made by the teeth onto a control card, in the box corresponding to the control being visited. When a control is expected to receive many visitors, as when a control is used by several different courses, several punches, all with the same pattern, are used.

At the finish line, the orienteer hands the card to the organizers, who verify that the punch patterns correspond to the correct controls.