There are a few ways to tell if polyurethane is dry. One is to touch the surface of the wood. If it feels tacky or sticky, then it's not yet dry. Another way to tell is to look at the finish. If it's still cloudy or wet-looking, it's not yet dry. Finally, you can try the "nail test." Simply press a nail into the surface of the wood. If it leaves a dent, then the polyurethane is not yet dry.