There are two ways that you can check your speed accurately. As I edited above, with a stop watch and GPS, on your phone.
For the stop watch, if you know math, you just set your speedo at any MPH, hold it steady and time between mile markers. Most that I have used (West Coast) will have 5, in a row. 5 is more accurate but, just one will be close enough.
For convenience, we'll use 60 mph. Set your speedo on 60, hit the stop watch at the marker, hold it at 60 for all 5 miles. It takes 5 min, to go 5 miles. Covert your min and seconds, in to all seconds, divide that, into 300 (5 min = 300 sec) then multiply times 60 MPH and you have your exact speed. For one mile it will 60 sec, divided by measured time.
i.e. 4min 33 sec = 273 sec, 300/273 = 1.0989 X 60 mph = 65.93 mph. For 1 mile, 60 sec / 53 sec = 1.132. 1.132 x 60 mph = 67.92 mph
You can do it with feet per sec, too (lots of time to think when you on a lot trip).
Measure your time, in seconds, divide into 5280 ft (or multiples of that) that gives you the number of FPS, you where traveling. There is 1.46666 (1.47 if you like) FPS in one MPH.
i.e. 5280 / 53 = 99.623 fps, 99.623 / 1.46666 = 67.92 mph