You need to travel over a known distance and compare the results from both the sensor and the phone's GPS-- there's absolutely no guarantee that the GPS is more accurate, as GPS is best at placing a stationary object, but can be off by 30+ meters while in motion. This is why people running the phone app can KOM Strava segments, as their segment that day was a hundred meters shorter than everyone else's.

I know that the SART has mile markers on the ground, the LART probably does as well. My Wahoo speed sensor reads 1.03 miles for every mile marker, yet if I do a ride with two computers running, one paired to the sensor and one on GPS, the GPS can be as much as 5% shorter on distance. Also, the auto-calibration with Wahoo may be as bad as Garmin's. My Edge 520 was set to auto-calibration, and shorted me around 4% of my total distance for over 2 years.

Strava segments are done by time over distance, so no matter what the speed sensor says, we're at the mercy of the GPS. I think my speed sensor over-reports about 2%-- so I get a "bonus" 200 miles out of every 10,000. I'm not really worried about it.