In this post about
ways to check your work, the artists responding to the poll were equally divided on two methods to check work. One method is looking at the work through a mirror and the other is looking at it through half closed eyes. I mentioned another method that I use to check and make corrections to work, and I can see from the comments that it is not uncommon to use a digital photograph as part of the process. In this underpainting that I've been developing for a Great Marsh painting I can illustrate how I've been checking and correcting work digitally.
This first image is the very first layer of underpainting.

So far so good. This painting is going to be as much about the clouds as it is about the marsh and I went in to add more detail and depth to the sky, while also adding detail and darks to the ground to balance.

I'm not unhappy with the results, but I can't help wondering if I might like it more with darker tones near the tree line and in the clouds. The surface is not dry enough to add a wash to it so instead I take a digital image and add the changes with Photoshop.

The changes are subtle, but it gives me the information I need and soothes my impatience at the same time.
I use this method frequently, finding that it's especially helpful and time saving on larger work.