It can be hard with current technology. In WW2 the Germans never suspected that the british had developed microwave Radar. They even had a Cavity Magnetron tube from a crashed H2S set on a Lancaster Bomber.
Conclusion they thought it was a hoax as they had no idea of the theory behind it and didn't even consider microwaves as a possibility.
Just before the war the Germans sent an Airship on a series of flights up and down the North Sea packed wit hdsetectors for Radio and Radar transmission. Germany was using rotating antenna with a wavelength of between 1 and 5 meters for it's Radar experiments. In the UK a complete system was already operational using 11m wavelength (this was chosen as half the wingspan of a Heinkel 11 bomber) Instead of using a rotating antenna CHL used phase shift between 4 transmisson and 4 reciever towers to give direction. It operated on a pulse rate of 25 per second, this was a sub multiple of the mains power frequency and was chosen to stop interferance.
When the Germans detected these signals they were so different to their experimental Radar sets they assumed they were just getting interference from the National Grid.
Conclusion the UK didn't have Radar.
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