What are the typical signs of bad gas or bad with too much water?

I was down to 1/4 tank and the PRV just had a heck of a time putting
out power and idling. Idle jump up and down (maybe the anti-knock was
kicking in). No misfires, also check that each cylinder was firing by pulling
the spark cord on each cylinder as the engine was running.

White and carbonized exhaust. No coolant loss or smell of gylcol
in the exhaust, it was just water (steam) from what I could tell. The motor
is retaining its oil, no oil burning or loss in sump.

I stuck 20L of fresh 91 (non-ethanol) and 250ml of 99% isopropyl . Ran it through
and all seems fine now. Warmed the motor on idle and mid-idle (2.5K rpm). Then a
short test drive around the farm and it accelerated like normal.

Think it was just bad gas? I don't drive this car much, once a week for 1/2 hour
to get pizza or Costco run.

My first thought when you said 1/4 tank was "in tank pump" but it does sound like a bit of water in your tank. Water being heavier than gasoline it doesn't make much sense that it only starts to happen once your tank gets down to a certain level, so if it happens again at the same point I'd check the pump. Otherwise load the tank up with some IPA and burn it right empty before filling again (carry a full jerry can).