After removing all of the hoses from the shaft, and removing the shaft from the base, plug up all of the hose connector openings (not necessary with auto-seal hose connectors) and the hole in the downstem. With these holes fully plugged, attempt to blow air through the top of the shaft (where the bowl sits). If your hookah is airtight, no air should pass through the hookah. If air escapes, you may be able to locate the leak by listening for hissing air.