If a wooden surface looks like it already has been finished with a product it is always a good idea to find out what that finish is as it may affect which product you should apply to the wood. For example, you could not really apply an oil over a varnished floor, but an existing floor that has been oiled can be re-coated with an oil (once it has been cleaned of course!). Here's a few pointers to find out what your wood has been finished with:
Waxed surfaces If it has wax on it you can scrape it lightly with your finger nail (once it has been cleaned). You may get a little wax under your nail. Also you can try buffing it with a cloth. Most surfaces will become shinier once buffed, but a waxed surface will substantially increase in shine.
Oiled surfaces Generally speaking oiled surfaces are not that glossy. If you stand something on the surface of an oiled surface it will not have much of a reflection. Instead you are likely to see a fuzzy image reflected. You can also try putting a little white spirit on a small unnoticeable area and if it dissolves the surface coating then it is likely to be an oil, please note however that this may leave a white spot. White spirit will also dissolve most wax polishes so check the above advice (waxed surfaces) before trying this.
Varnished Surfaces A varnished surface will be hard to the touch (it will make a clicking sound when you knock your nails on it) and will have a reflective surface. Even a matt varnish will reflect the light more than an oiled surface. If something is stood on the surface (with a window in view behind it) then the reflection is clearly visible. A satin or gloss varnish will have a very clear reflection of anything that is held up to the surface.