All sprinkler systems are required to have indicating control valves. Indicating valves physically show whether a valve is open or closed. A closed valve indicates that no water will flow beyond the valve and serves to alert the observer to the situation. In the event of a fire, no water would reach the individual sprinklers.
You may sometimes find a control valve sealed in its open position. You can seal a valve by passing a wire and lead seal (or a similar device) through the hand wheel and a stationary object on the valve. A broken seal indicates that the valve has operated after placement of the seal.