Leakage is harder to see if light is blocked from entering the back of a stone.
The more enclosed the setting the less back light. Aim for good even light from behind the stone, but beware of direct glare; it will make facets look pale and shiny.
Light leakage often occurs in the upper girdle facets which may be covered over by the setting. If there is leakage in the centre then there is usually leakage in the outer facets.
Dirt and Grime
Even though this diamond is not well cut, you can see that dirt gives a misleading impression of leakage. Clean first with very hot water, ammonia and detergent and a good scrub with a toothbrush.