Stepping back and looking at your painting in progress from a distance is very helpful, it makes you see with fresh eyes. The different point of view lets you see the big picture and the main shapes, without getting caught in the details.
As an alternative to stepping back, you can look at your painting in a mirror. The reversed image reflected in the mirror lets you see things that you may not notice looking straight into the painting.
Another thing that helps is taking a photo of your painting and then looking at it on the screen. Many times I take a photo to post saying it's done, and when I go to post I see things that jump out at me as still needing attention.