A good paint job is one that is done on time, with good quality paint and materials, doesn't seem rushed and leaves the house free of paint fumes and debris. Check if brushstrokes are visible around the line and the edges of the ceiling. Check the whole house because the brushstrokes will remain visible forever. If you can see the strokes, it means they didn't apply enough paint. They provide information about the quality of the painter's work, its punctuality, cleanliness and how they handle problems if they arise.
When talking to references, ask them about their general experience, the quality of the work, and if they would hire the painter again. You'll get more preparatory work, high-quality paint materials, and the most experienced painters will take care of your entire house from top to bottom, inside and out.
