By checking if it grinds.
Let's think about the mechanism of a grinder. Why grinders may get damaged if you adjust them when the burrs are not rotating?
A perfect coffee grinder design is a very serious engineering problem.
You need to have two very rigid burrs. When you don't have this rigidity, burrs may be damaged under high pressure of stuck coffee beans. Also, you don't want them to change their ideal shape in time.
You want the burrs to rotate in opposite directions when they are so close to each other. The distance of the burrs must be equal at every rotational position to produce consistent ground. In practice, every grinder is a bit skewed from the ideal position. So, burrs are closer to each other at one angular position and more apart at another position.
You want a heat conducting material to keep the beans from burning after all the friction in between the burrs. Also, you don't want the heat deposit on the burrs.
The caution you've noticed is mostly relevant to the second bullet. If, by mistake, you squeeze the burrs so close to each other they may damage each other in the next run. So, they can grind each other. While the grinder is running, even if you want to, you cannot squeeze till they touch. Even if you do, you suddenly hear a loud noise to release the setting a bit. However, if the grinder stops at an (somewhat) apart position and you squeeze them to touch, you cannot notice that.
If it grinds consistently, probably it is OK. But do not gamble again.