Bright light reflected off a facet can make it easier to see surface characteristics and help you determine if an internal characteristic breaks the surface. This isn’t the only way to find surface-reaching characteristics, but it’s very helpful.
With a microscope, use darkfield illumination to search for and examine inclusions. Darkfield illumination lights the diamond from the side against a black, non-reflective background. It lets you see inside the diamond and causes its inclusions to stand out as bright objects against a dark background.