Years ago I was involved in gliding and flying distance was very important as that was the " racing " of the day ie how far in X amount of time. GPS loggers appeared on the scene and a large number of the pilots who I thought were pretty good by their discription of where they went, all started having really bad days.

Now GPS units are almost a dime a dozen so simply ask for the trace off the GPS they almost certainly carried, in fact ask everyone to carry one and download the traces onto a laptop, over laying the records are quite a good learning device and does tend to keep people honest.