one suggestion is if you happen to see one of those big speedometer things along the side of the road that tells the motorist when they are speeding too fast. You could try the machine out when it is not too busy and compare the result to your computer.
around here we have a speed limit sign with a built in speedometer and another one of those big machine on another roads with plently of space for a cycler to be detected.
Just a idea if you happen to spot either one of those maching on your test ride. if you are still not sure, you probably could ask ad see if you could have a friend with a computer equipped bike ride alongside you so you can compare and adjust your sensor to match the friend's computer speeds. (though if the friend's computer aren't set-up right, this mean your's will not be set up right either)