The source
Firstly, you should check to assess the root area of leakage. Make sure to check for the roof leakage only when the surface is cool and dry. If the flat roofing is wet then it will be very difficult to see any leakage on it. The susceptible places for leaks to appear are the flashing, which is over the join of roof parts. Make sure to be meticulous in your inspection of the flat roofing. Check to see if the pieces in any place are joint loosely or if there are any cracks or gaps, which have formed on account of wear and tear. The flashing should also be ideally integrated in such a manner that rainwater moves away from it. This is the best way to keep your flat roofing free from water damage.
Low-lying areas
Sometimes, the flashing may appear in perfect condition without any crack or gap in place. If you are unable to detect any flat roofing leakage on flashing then check on lower roof areas. These regions tend to accumulate a lot of water and hence can develop cracks and corrosion easily. These resemble dirt rings with each ring diminishing in size as it goes down below. Check on the lowest areas of the flat roofing to check for holes, cracks and splits on them. This can be a major cause for water leakage and thus you need to inspect these areas meticulously.
Roof seams
One other place where you can find your flat roofing developing plenty of cracks is the seam area. Whenever there are joints between flat roof parts there will be weak linkages in these spots. Thus, these areas are automatically more susceptible to leakages and water seepage in later years. As your flat roofing gets older, you will find more water seeping into these roof seams resulting in severe damage. Even if you happen to seal up these seams well it can still have leakages in them. Therefore, always be sure to inspect the roof seams thoroughly when you are looking for leakages on your flat roofing.
Using a hose
If you have inspected all of the above mentioned areas and been unable to detect any leakage on your flat roofing then you can use another method. In this method, you should take a hose along with some water and start with one section at a time. Make sure to allow the water from the hose to wash over the section of the flat roofing uninterrupted for 15 minutes or so. The moment you see the water flowing towards leakage source, you have found the spot. Then, you can note it down and proceed to seal it up.