There are two major categories of hardwood floor finishes. Knowing which type graces your floor will help determine how best to care for it.
Unfinished solid hardwood flooring has a penetrating finish, which does as the name implies; it is absorbed into the floor, and then topped off by a coat of floor wax.
Found on prefinished solid hardwood flooring, surface finishes, such as polyurethane, form a protective layer over the floor, and are some of the most common finishes in use today.
Run your hand over the floor surface. If you can feel the wood grain, the finish is probably a penetrating finish.
Select an inconspicuous area of the floor surface and apply a small amount of paint remover. If the finish bubbles up, it is most likely a surface finish.
With a sharp blade (also in an area hidden from view) try scraping up a bit of the finish. If you are successful in scraping up a clear material, the finish is likely a surface finish. If the floor smudges but no clear material is scraped up, then the finish is likely a penetrating finish.
Choose a high-traffic area where the finish is likely to be most worn and pour a tablespoon or two of water onto the floor. If the water beads up so that you can wipe it up without a trace, your floor's finish is doing its job.
If after a few minutes, the floor slightly darkens because the wood has absorbed the water, don’t panic. Your floor's finish is only partially worn. Immediate refinishing probably isn't necessary, but keep a close eye on it.
But if the water immediately disappears into the wood, leaving a dark spot, your floor's finish is seriously worn and it's probably time to recoat or refinish.