You could measure the amount of time the vehicle takes to cover a certain distance at full throttle. With a running start (i.e. max speed throughout) you should be able to roughly calculate your speed - although judging when the car is passing a certain point at some distance away can be difficult, making the results inaccurate.
You will need help of two other persons. (flag men) lets say you have 50 yards of distance(example only). #1 will be at 25 yrds and indicate start of timer, #2 will be at the 50 yrds and indicate stop. The car build's up speed from 0 - 25 yard and likely to be full speed by this time already. Driver could be somewhere close to the 50 yrd marker.
Offcourse everything will be a rough estimate, due to the reaction time for the person to raise their flag, and reaction time to start and stop the timer. But you'll have a good idea how fast you are going.