The usual way is to do a water test. the basic tool is a water hose. You start below the suspected area, and slowly work you way up the slope while sweeping the water stream from side to side. Each spot should get 5-20 seconds of water on it before you move on. If flashing is suspected to be leaky, pour the water directly onto the area where the leak is suspected.
Of course, using this method, a second person should be on the inside, watching for leakage. If the leak point is very small, you may never see a leak since it takes a long time for the leak to build up a volume of water that's visible.