Free, and it never uploads
Two masters. One honest comparison.
Drop two versions of the same song — the one you were sent and the one you already had, or two quotes from two engineers. They play at matched loudness, because the louder file always sounds better until you take that away.
What this measures, and what it cannot tell you
Everything here is measured from the audio itself: loudness, true peak, range, tone balance, stereo width, and the marks a lossy encoder leaves behind. Those are facts about the file.
Which master is better is not a fact about the file. A quieter, more dynamic master is right for one song and wrong for the next; a brighter one suits one mix and fatigues on another. This page tells you what is different and what each difference will do on a streaming service. The choice stays yours.