Don't mean to sound like a wiseass, but the only "tell tale" signs of ducks is seeing ducks in the spot. With that comes scouting, and lots of it. If you have access to flooded timber I would say you are way ahead of the game, especially looking for woodies. Look for open pockets and/or trees that will drop acorns in the fall. Should be able to find a fair share of mallards in there too if its producing any food. River spot sounds promising...if it has any slack water in close proximity that you can hunt as well you should be in business. Ponds will be the easiest to scout and you can do that with binoculars most likely. Scout it out and make it happen for the opener!