Well the person who supplied the one in photos listened to it before doing so. Presumably the person who is supplying the one being measured did so. And no reason an ear test can't be done.
Now you asked what will the test show. I expect they will test frequency response, distortion, and noise over up to 20 khz. They may do more than this. If the results are the same with or without the device it does nothing audible. They may also do a null test, but I'll say no more as I am not the one doing the testing.
Now you are wondering why not a listening test just to see. For one thing, if a device measures the same on all the above tests it has been found to sound the same. Lots of claims different, but they just don't pan out.
Now another issue is people are just prone to hear a difference on the flimsiest excuse. Not that they are making it up or aware of what is happening. Some fair number of people just knowing they plugged something in may honestly try to see if it sounds different and find that it does. Even when there is no reason for it. They are even more likely to have the experience of hearing a difference if someone has primed them to expect a difference.
The way to deal with that is blind testing. Someone plugs in and alternately bypasses the device while another person listens and says whether the sound is different or not. Devices that measure the same, but people listen to and swear how real the difference is turn out not to be discerned when the listening comparison is done blind.
It is good to be aware of how easily your ears/mind will trick you when you know something has been changed. It happens to everyone, and no one is totally immune from it no matter how hard they try. Very hard to get many to believe this. Hey, why are you so close minded they say, just listen for yourself and see. Such a situation anyone is very likely to listen and hear a difference that is not really there. Just trying something out to see what you think seems so simple.
Now if the differences are large, then yes you can actually hear something is different with a more casual approach (though matching levels is important even here). But anything so easily detected with sighted listening will measure obviously different. The tricky issue is differences get small enough our ears may get tricked well before measured differences get too small to be detected. By then your brain is likely to grab hold of nearly anything and match a pattern to hear a difference even when there is no difference.