My test was this. I took note of my co-ordinates. I flew for 1 minute on mount (I have 310% speed paid for) I noted the destination co-ordinates after 1 minute. I then went back to the start pointed in the exact same direction (using a remote point as line up reference) and flew in flight form. I then noted the time it took to reach the same co-ordinates.
I could also verify the speeds using the co-ordinates, and standard speed=distance/time formula for both journeys.