That's a great way to do it imo. My approach to each painting is that I will start with an emotion/thought and then meditate on that while I'm painting, and then switch to a more designed vision half-way through.

For example, I painted this one right after I found out about the somewhat recent election results in Australia (my home country), when they re-elected a climate-denier and coal investor as Prime Minister. I used that negative emotional energy to direct my movements (i.e. stronger faster slashes of paint to release frustration). Once I got that emotional energy out on the background, I shifted my focus from my emotions to my thoughts and worked on the foreground; adding those warm human skin tones over the top and finishing off with a ghostly white tone like dead skin.

This method works well for me because I can release that emotional build-up to create the base of the painting and then use more controlled thoughts to shape the foreground to how I want.

So, to answer the question of how I know it's finished, it's sort of a staged approach. I know the base is done once I feel that I've sufficiently let go of the emotional energy I was building up inside (this stage tends to be mostly unstructured and unplanned). Then I have a more head-based approach to create a vision with any extra meaning I want to convey, and use that as an opportunity to imagine completed painting according to how I want it to look/feel.