If you're hunting public look for areas off the beaten path or that are hard to get to. It's been my experience that public land hunters are lazy. If they can hunt a spot that they can boat up to, throw out decoys, and hunt out of the boat they will.
Look for spots holding ducks in different conditions. Spot A seems to hold birds on a strong north wind; spot B holds them when it switches from the south. Give yourself plenty of back up spots.
+1 on finding the feathers. If ducks are preening then they are content; if they are content you don't need much to get them back there.
One scenario I experienced last season: found a couple of big concentrations of ducks right before the season in a long walk in area that had just gotten lots of rain. At first glance, it looked like they were just randomly sitting in large flats of shallow water. After a few hunts and other hunters moving them around the area got more hit and miss. Ducks were still there, but it was tough to pinpoint exactly where they wanted to be in this huge area. I finally made the couple mile walk all around the area and determined that every small group of ducks were sitting in almost the exact same depth of water. Looked at old Google Earth aerials and saw that this area used to be ponds, but now with the sheetwater the whole area looked to be the same depth. From then on if there were birds using that area, you could set up on almost any pocked of water with that depth and had good hunts. Sometimes there is more going on than what you first think. You can't really beat getting out and walking around.