1. The proof that it was fermented wine: it was a wedding!
2. Weddings lasted a long time, as this one seemed to (that doesn't happen without real wine).
You obviously dont drink wine, or know enough about wine, no one 'really' talks, or gets excited (as the master in wedding did) about the goodness and flavor of more grape juice, and its too sweet a juice to be refined in taste (and I have made my own grape juice). Fermented wine has a world of its own, 'taste' is more than a thing with wine, and the good, or best wine is something of the gods, and in this case God (I guess you don't understand)
3. You did not study the word
methysthōsin (John 2:10), the same word is used in at least seven other senses and it always means 'drunk' of some sort;other verses with the same word;
and eat and drink with drunkards...
the woman drunk with the blood...
For these men are not drunk, as you suppose..
who get drunk get drunk at night...
earth and were made drunk those that dwell on it...
envying, drunkenness, carousing...