The wood had to have been treated with a sealer or it would have turned grey by now, even if it had been stained. I would pull a board out and do any sanding/testing on the bottom side. Take it to a good Porter Paint or maybe a Sherwin Williams store and they should be able to tell you what the wood is as well as help you with deciding what you want to do to refinish.
We do a lot of "deck restoration" work here...everything from redwood to treated lumber and I can tell you that you will NOT be able to just stain/seal it as is without either stripping the old sealer off or sanding it back down to bare wood. What I'm saying is that going over the old stuff won't hold up worth squat...you have to get the old stuff off.
