[pgeorgi]

Usually it's double blinded ABX testing: A computer program gives you: Encoding A, encoding B, unknown encoding A or B. You have to choose if X is A, or X is B.

One of the encodings will be "uncompressed" when testing if the format in question is perceptually lossless.

Repeat that a couple hundred times with many different listeners using standardized samples and programs (doom9, an audiophile forum, does such runs every now and then), and you get a rather good idea on what's going on.

As for the 192kbps: It also depends on the algorithms used. bladeenc or 8Hz-mp3 back then created 320kbps files where you can easily hear the difference. Current lame builds at 192kbps? not so much.