How do you determine the speed of an airplane from the ground with nothing more than a stopwatch?

In the 2004 Martin Scorsese film, “The Aviator”, there is a scene where they re-enact the1935 speed test of the H-1 Racer. The only instrument they use to measure it’s speed, is a stopwatch. How is this done? Are there markers or points of reference on the ground, that the plane pes over, that I didn’t notice, while watching the film?