get a watering can or similar and some food colour, mix with water and pour around a little at a time on suspect areas on roof while checking below , if too flat build a bund or circle with sand ,moving in small areas till you find the leak , watch out the colour doesnt stain wallpaper or ceilings , use just water if staining can happen ,water can enter and run down beams etc exiting some distance away so the leak could be elseware than you expect wash colour away with water
Mustangned NZ mustangned at gmail.com, January 2011