I think most of the time they stick the device in an anechoic chamber and take measurements at certain distances. For example, under the CISPR 22 standard (that information technology equipment with a supply voltage of less than 600V falls under) it must radiate less than 30dB (uV/m) of electromagnetic radiation between 30 to 230MHz, measured at a distance of 10 metres with a quasi-peak detector, less than 37dB at 230 to 1000MHz, and so on.

There are also restrictions of how much interference is caused from the power supply (which shouldn't apply to the Raspberri Pi) and how much EM is emitted from telecommunications ports on the board (which will apply to Ethernet, but probably not USB).