As an engineer, I revisit this topic regularly. It does not help that I am now older and require reading glasses for close in.
It is good practice to have a general idea of DoF for focal length and apertures before ever going in the field. This is not for precision, but to quickly eliminate options. For example, you can look at a scene and the light and the focal length and just say "it ain't gonna happen" and move away from trying to get everything in relatively sharp focus and position the DoF to where you want it.
Using the viewfinder: I use a magnifying eyepiece on my Nikon D810 to help and I still have a dark cloth that I can shield my viewfinder/eye with in bright sunlight to use the DoF. Takes a few seconds for your eye to adjust to the reduced light but it works.
Live View: The magnification option of live view is fantastic for this. However, you will still want the dark cloth or some purpose built shade/hood for the screen when in bright light.
And Armand hit on my biggest peave with camera manufacturers. I won't generalize, but as I Nikon shooter, every single modern Nikon with a "D" model lens or newer on it knows the Focal Length, Aperture and current focus distance and could report to the user the Near Focus Point, Far Focus Point and the hyper focal distance. There should be CoC setting in the camera for the user to choose.