Start with the easiest clues:

• Catchlights and reflections: Look in the eyes, glasses, or shiny surfaces. These often reveal the number, direction, and sometimes the shape of the front light sources.
• Shadows: Check the nose, chin, and facial contours. The direction of the shadows shows where the light is coming from. Multiple visible shadows can suggest multiple lights, though fill light may make them faint.
• Shadow edge quality: Crisp, well-defined shadows usually mean a small or distant light source. Soft, gradual shadows suggest a larger, more diffuse, or closer light source.
• Rim or halo light: A bright edge around the hair or shoulders often indicates backlighting or a rim light used to separate the subject from the background.

You usually can’t determine exact distance or modifier type with certainty from a single image, but you can make reasonable guesses from shadow softness, catchlight shape, and how strongly the light wraps around the face. In practice, analyzing reflections, shadows, and edge highlights will get you most of the way there.