Polyurethane, shellac, varnish and lacquer: use a cotton swab with acetone and apply it to the wood. If it gets sticky, then it's shellac or varnish and if it forms balls, it has a polyurethane finish. If it is lacquer, the lacquer will completely dissolve. Change the solvent to a lacquer solvent and repeat the first step by applying a few drops to a new area of the surface.
If after a couple of seconds the finish softens enough to almost flow, you have lacquer. You know it's not shellac. If you have wooden furniture that needs finishing treatment, it's a good idea to highlight its beautiful wood grain instead of painting over it. To do this, you will have to seal the wood so that the surface is exposed and protected.
To properly seal the wood, first prepare the surface by gently sanding it and, if desired, staining it in a color. You can then apply a sealant, but there are plenty to choose from. The three most common sealants are polyurethane, shellac and lacquer, and each requires a different application method. To try to figure out the best way to seal and protect it, I bought a sample of wood from Restoration Hardware to test it (transparent polyurethane) and (Waterlox) and see how they affected the appearance of the wood.
To find out what type of finishing treatment your furniture has, you can try a discrete area of the wood, such as the upper inner part of a leg. Not only do finishes protect wood from spills, stains, surface abrasion and moisture changes, but they can also enhance the beauty of the wood.