https://www.youtube.com/watch?v=4_4XLeCmLak

If you have a camcorder and fly, you can easily verify the rough speed of the plane you are in.

This is how.
A helper would be nice to measure the time when you say start and stop.
You need to hold the camcorder in a steady position as you look out of the plane's window.
Find some kind of big enough landmark not far away, lake, object, structure that has distinctive borders you can start and stop your time measurements.

If you would have a grid on your camcorder view finder it would make the measurement a lot more accurate.

As your plane flies by this landmark, move your camcorder view finder to the edge of the landmark you selected. Line up the left side of your view finder with the edge of the landmark. The moment you do, tell your helper to start timing. DON'T MOVE YOUR CAMCORDER, LET THE PLANE MOVE IT! MAKE SURE THE DISTANCE YOU MEASURE IS PARALLEL TO YOUR OWN PATH! When the left side of your camcorder gets to the other edge of the landmark, as the plane moves, tell your helper to stop the time count.

Try to conform your measurements to a parallelogram shape, as close as possible to increase accuracy.
Now you have the time signature that was needed to cross that landmark at a certain speed.

Go to google earth, find that landmark you selected and measure the distance from edge to edge of that landmark that was timed to cross when you were on the plane.
Now you have the distance of that landmark you selected.

From the formula, Distance = Speed X time, you can calculate the speed.
Speed = Distance / Time
Now you KNOW how fast your plane was moving.
Do several such measurements to increase your accuracy.

I often did this kind of measurements when I was flying. Focused my eyes on a certain bigger object, structure, landmark and watched the speed how fast we were passing over it. It didn't look like we were flying hundreds of miles per Hr but something like 100-200 miles per Hr. Never really paid a great attention to it though. IF YOU DO THIS, YOUR VIEWING ANGLE CANNOT CHANGE AND YOUR VIEWING “line” MUST BE PARALLEL TO YOUR COURSE.

You could also use a reference on the bottom edge of the plane's window and focus on that, while your landmark will be blur in the distance. Same idea as when you are aiming a rifle. Just make sure the plane is not turning.

On the ground, using trigonometry, google earth, flight radar data, time measurement, angle finder and some helpers, people should be able to do a ROUGH speed (plus/minus 10% of real speed) measurement check of commercial planes, especially if one flies directly overhead.

If you mark a plane's position in the sky, (preferably when its track is going to be overhead) and measure thirty degrees from that marked point and time how long it took for the plane to cover that thirty degrees by two Pi X the listed altitude of the plane from flight radar site, you can calculate the imaginary circle of that plane's path (if you are Heliocentric).

Divide that imaginary circumference by twelve and you get the distance the plane flew in that thirty degree window. Now, from the time and that distance calculate its speed.

IF the planes are not flying as high as it is claimed by flight radar, obviously, the plane covers LESS distance. If the distance the plane travelled is high, your calculated speed will also be high and it might match the listed speed of the plane on the flight radar sight. The goal should be to eliminate "their" data and rely only on yours.

The Sun, is only a REFLECTION from the reflective ice sheet (molten looking glass). That's correct. An “optical illusion”.
https://www.youtube.com/watch?v=vwQ2nyIFkso