I see that "official" records for land speed started in the early 1900's and given the technology available, how were the times verified?

Devices in cars, optical traps, radar all don't seem available, possible or verifiable.

Also, I am aware that you can calculate average speed with a simple calculation but not top speed - and to be honest that still points to two people with stopwatches.

So my question is basically about early 1920 times where the cars were starting to go pretty fast.