Water flows downhill so find the lowest spot using something round like a ball. It should keep rolling to the lowest point. Chance are the leak is around there.
With the help of binoculars we can find out where a flat roof leaks. Find out the cracks or tears, then clean the area properly. Then apply two or three layer of cement.
On a flat roof the professionals sometimes take a basketball and place it on the center of the roof. Normally the ball will roll to the low spot within a few seconds, this is where the leak most likely originated.
