This test is a matter of setting up two identical microphones, into a mixer, one connected with an audio cable and the other with a wireless system, to perform a listening test. An even better way to perform the test is to split a single mic signal so that one path is through the wireless mic system to the mixer and the other is hardwired to the mixer. Though seemingly simple, there are several mistakes that can really mess up the results:
Switch back and forth between the cabled and the wireless setup as the listener compares the sound of each setup. This, of course, is best done in a “blindfold” manner where the listener has no way of telling which setup is being monitored, and by writing down a few notes about the results.