If you're reducing for a sauce, you're looking for nappe. It coats the back of a spoon, and when you draw your finger through it, it leaves a distinct line.
Nappe is French, from napper, to coat. It's worth learning the word rather than the description, because once you have it you can use it on any sauce for the rest of your life.
The test
Dip a spoon and draw a finger down the back of it. If the line holds clean, you're there. If it floods back together, keep going.
It has grades, and that's what choosing a viscosity means. A light nappe for a delicate piece of fish, a heavier one for something that has to cling. Same test, different stopping point.
Reducing by half is a different measurement
Reducing by a fraction is about concentrating flavour, and you judge it by volume in the pan. Nappe is about texture, and you judge it on a spoon. A recipe usually wants one or the other, and knowing which one you're chasing is most of the skill.
The one way this goes wrong
Stock reduces, salt concentrates. And it's almost never your hand that does it: ordinary store-bought broth is about 0.9% salt and reduces to roughly seawater. Same for soy, miso, anchovy, a parmesan rind, cured pork, or the fond off a protein you salted an hour ago. That's why classical stock is made unsalted.
So salt lightly, taste the whole way down, and correct at the end. Season to the volume you're going to finish with, not the one in the pan now. An unsalted reduction tastes flat all the way through and gives you nothing to steer by.
There's a second failure and it can't be fixed. Over-reduced wine turns harsh and bitter, and unlike salt, nothing you add afterwards corrects it.
When you're not really reducing
Anything thickened by starch, egg or dairy instead of by evaporation. A roux gravy, a cornstarch slurry, a custard, hollandaise. They all reach a coating consistency without reducing, so the frame is wrong even when the texture looks the same.