Several ways to do this, but I guess the metod mention above was most likely used.
As a kid, I found out the speed of my Daisy Winchester pellet gun by shooting at a dry tree with the help of my dad clocking the time from the bang until we heard the impact.
With a distance of 83m, we knew that it would take the 1/5th of a second for the sound to reach us. And by that I found a fairly accurate estimate
JorgeO, DVD was sent today btw