Good question and a tricky topic. Short answer, I try to view the print under as many different lighting conditions as possible (except direct sunlight). The lightbulb in my darkroom is rather bright and I've ruined many prints by using it as the sole guide. Now, if I print during daytime, I always view the test strips (microwave dried) and prints by a large window. I might even carry them out onto a shady balcony. When printing at night, I try not to view the print directly under a glaring bright bulb, but keep the print a couple of meters away, which - I hope - should give me an OK look by window light.