Ok, so imagine then that I have an original .wav file at full resolution, let’s say 16 bits 44.1kHz. I then take that and make it into an mp3 at a pretty low rate. Then I bring that back into Cubase or Nuendo and resave it as a .wav file at 16 bits 44.1kHz again. We now have the original “A” and the new file “B” (“B” is not a “copy”, so we should use a different word probably).
If you want to tell the difference between the two then the first option is to just listen. You’d often hear a difference.
Another option would be to look at the file using a spectrogram. It’s possible that the file that went through lossy compression got a bunch of stuff thrown out and that stuff wouldn’t come back. So you could possibly see a lack of high end in that spectrogram when comparing the two files.
It’s also possible that loudness and peaks have changed. As for “distortion” I wouldn’t be entirely certain that you would see that. I think it’s possible, but not necessarily the case.
Normally if I brought that stuff into software to analyze I’d guess that whatever has a more extended frequency response is the original. It’s not always true I think, but most of the time.
Was that what you were looking for?