The only way to be sure is to travel through a known speed trap at xx miles over the posted speed limit and see what the LEO says you were traveling, according to his/her radar. A citation, fine and insurance increase may come along for the ride too, so consider deeply before trying this method. I have a 35MPH reader up the road from me, and the C14 reads 37, when the read out is 35. I have searched for a readout above that speed, but haven't found one. Asked a local LEO, and he wasn't aware of one either. It's a conspiracy, I know it is. The city wants my money.... tp. 
Italians can help with an explanation here...
I drive quite a bit in the Italian Alps because their Austrian counterpart are so much less flexible about speed limits. Anyway, the Italians needed money and started installing the so called Autovelox. But before getting there, which is simply a money trap, 10 years ago they were actually concerned with safety, so started installing those readouts.
What happened was that those insane Italians started using them to settle discussions about who was doing faster. You can see where it was going...
So the police changed the readouts to stop displaying speeds above twice the posted speed limit.
Now they are on the Autovelox, and fines ARE being sent across the Alps
Don't ask how I know...
As a last twist on the tourniquet to choke the fun off the roads, they now copied the Austrians with the Tutor system, which tracks vehicles over a certain stretch of road (several km/miles long) and calculates the average speed. Neat...
This is the reason why I always praise a CC or at least a throttle lock of some sort.
