Place a few drops of water on top of the wood floor in a few different spots, especially areas where the wood looks more worn.
Observe the water for a few seconds. If the water soaks immediately into the wood and darkens the wood, the fibers are exposed and the wood requires resealing.
Stand back and look at the finish of the wood. If it has lost its shine and looks dull and dingy, it needs to be resealed.
Inspect the floor closely for scratches, gouges, chips, cracks or stains. If the flooring has more than a few noticeable spots, it's time to seal the floor and possibly repair it as well.