Taste it as you go. If you feel astringency at the back of your throat, the alcohol hasn't cooked off yet. Keep going until it turns round and caramel.
Almost every other test in cooking is something you look at, listen to or press. This one is a place. Feel it once and you have it forever.
It can't be confused with acidity
The two work on different systems. Sourness is a taste. The alcohol grip isn't a taste at all, it's closer to touch, carried on the same nerve that reports chilli heat and menthol cold.
You don't need any of that to use it. Acid makes your mouth water. Alcohol makes it dry and tighten. That's two independent tells, and they don't feel alike once you have noticed the difference.
The myth it replaces
The usual instruction is to cook the wine for a minute or two and call it done, or to light it and assume the flame took the alcohol with it. Neither is a measurement and both are generous.
A flamed pan keeps roughly three quarters of its alcohol, because the flame only burns the vapour above the surface and it's out in under a minute. Fifteen minutes of simmering leaves about 40 percent. An hour leaves about 25. Two and a half hours leaves about 5, and it never reaches zero, because the alcohol and the water leave the pan together rather than in turn.
So the cue is reading a threshold, not an absence. When it drops below where you can feel it, what is left is the fruit, the sugar and the wood the alcohol was carrying.
Chase the positive tell
Alcohol burns high and turns bitter low, so the burn clears before the bitterness does. Stop the moment the sting goes and a thin bitter edge can still be sitting under the fruit.
This is why the cue doesn't end at until the astringency is gone. Keep going until it turns round and caramel. Cook toward the thing you want, not away from the thing you don't.
Where the test doesn't read
A marinade, a chilled dish, or a finishing splash meant to stay bright and sharp. And not vinegar, which is acid rather than alcohol, so this test doesn't read it at all.