When drawing reflective surfaces, careful observation is your most important tool. You will notice this image shows three types of reflections-- glass, metal, and a flat surface.
To begin thinking about shiny surfaces, consider fun-house mirrors. They are reflective the way metal, glass, or other smooth surfaces can be and they also bend in unusual ways. Reflections in the objects you will be drawing are very similar to images in fun-house mirrors. Like a mirror, the different tones and shapes you see on shiny surfaces are REFLECTIONS of whatever surrounds the reflective surface.
Neither metal or glass, this cup has a particularly glossy surface that clearly reflects the napkin sitting in front of it. Notice that in the reflection, we are seeing the side of the napkin that faces away from us.
On this cup the image reflected is the fabric on the left and fabric that is outside of the picture plane.
Drawing what occurs on a reflective surface requires special attention to how warped the image in the reflection becomes. Because this surface is rounded what is above and around the object is reflected.
M.C. Escher is a master of drawing reflections. Notice the distortions in perspective and his arm and hand. To help understand what you are seeing reflected you have to pay close attention to all surroundings in the room that could be in the reflection you are drawing.
This complex drawing reflects other objects within the picture as well as the kitchen that is outside of the picture plane.
Even the shadows of other balls in this image are reflected on surfaces.
Notice the difference in types of reflections on these balls. The metal ball in the center reflects clear images, while the glass ball to the left shows reflections of light sources.
Drawing reflections in glass is different than drawing them in metal. This is because glass is transparent. Not only does it reflect light and shadow, but you can also see through it.
One key to drawing glass reflections is to understand your light source. When drawing the surface the whitest white should be present as well as some pure black.
Like the distortions that occur on the surface of metal, objects seen through glass becomes distorted. To reproduce this there is no trick except careful observation.