In September in the Dakotas, most ducks are spending all of their time on shallow ponds feeding on pond weed, various invertebrates, and seeds. Mallards may start feeding in recently harvested silage, wheat or other ag fields.
In the middle part of the season (second half of October into November) birds are in a fairly predictable cycle of feeding mornings and evenings in grain fields. Birds may be roosting on bigger bodies of water and spend time between feeds on a smaller 'loaf'.
Late season ducks are typically doing two things, feeding and keeping water open. In the late season, corn is king and all birds are trying to consume enough calories to stay warm and keep their fat reserves up for the final migration.
The first step many sucessful hunters do when scouting is start on a computer. Today's online tools like OnX or Google Earth make scouting large areas much more efficent. Start looking for areas that have multiple sloughs in a few mile area, or maybe a small pond near a larger lake. Next, zoom in on a slough and check for vegetation on the surface of the water, cover around the edges, and the general size of the pond.
If there has been any substantial flooding, I focus all my energy on finding flooded fields. Often times you can look on the satellite images in areas with multiple ponds or sloughs or some areas that may have a waterway or creek running by them and see spots in fields where the crop hasn't done very well due to higher water. If there is a patch of grass in the field, there is a good chance that it doesn't get planted because it is too wet.
Since birds in the middle part of the season could be looking for flooded fields or harvested fields, I start by driving to the area of multiple larger bodies of water. These places attract migrating ducks and ducks will start to flock up on these waters. In my experience, birds seem to fly out to feed 60-45 minutes before sunset, so find a good vantage point in the area of cut corn fields and get ready! If you're in a good spot, soon you'll see a flock of ducks spiraling above a feed. If you do a few scans and don't see any ducks, its best you move 5 miles or so and repeat the process. Eventually you'll find what you're looking for!
The name of the game in late season is big water, warm water, or moving water. These are the only kinds of water that will stay open for ducks to use. As long as there is open water and an available food source, ducks are known to winter very far north. When scouting birds in the late season, It works well to find ducks on the water or in the air while flying out to feed. From there, you can follow the ducks to their feeding location. Once you found the feeding location, you may have to ask permission. If you are unable to hunt the 'x', think about other options that may be in the flight path of the birds. Resist the urge to hunt the roost if possible. Once the birds have been shot at in their bedroom, they are unlikely to stick around.
The image on the right shows a spot that would likely hold many early season ducks. It is in the vicinity of other larger bodies of water, but contains many smaller holes with obvious vegetation growth to provide birds with feeding opportunities.
The image on the left is a great example of an area with roosts, grain fields for feeding, and areas that ducks could use for day loafs. If I were in this area late October, I would find a place to park between the bigger lakes, get out my binoculars and cheese-its, and wait. These bigger lakes may hold ducks after the smaller sloughs start to freeze.
The image on the right is an example of a late season, big water roost. Bigger water cools more slowly so the bigger deeper lakes will generally freeze last. If a lake has springs or an inlet, the freeze will be delayed further. Even when ice begins to cover the lake, the birds will keep a hole open as long as the temperatures don't dip too low. Make sure that the water you are scouting has grain fields nearby. Keep in mind, if there is too much snow, the birds will move because they won't be able to get at the feed.
*BONUS TIP* Small Creeks are another good place to find ducks. They don't freeze as fast and can provide some unforgettable waterfowl action!