I know from experience that locating a leak in a flat roof can be very difficult and expensive.  In our house it took 3 different roofers to identify the place in our flat roof that a leak was coming from.  The reason why pin-pointing leaks in flat roofs is troublesome is that very often the point where the leak shows in the ceiling below can be quite far away from the place at which the leak is occurring in the flat roof itself.  

The best places to look for leaks on flat roofs are flashing or joints or seals of any kind.  Now, flashings are used at the point at which the roof joins non-roofing material such as brickwork.  For example above our kitchen extension we have a parapet wall at one edge of the roof and flashing closes the gap between the roof and the wall.  

As described at flat roof company, a capable roofer will first examine such flashings to identify gaps through which water could perhaps pass. Flashings are usually made of a metal material such as lead.  

It is usually difficult to test for leaks on a flat roof in the way you would on other types of roofs. It could take hours for water to trickle down to the ceiling from the moment you poured it on a suspect area on the roof. What this amounts to is that pratically, its very hard to test and then track multiple suspect places.  This is explained a bit more at the epdm flat roof site.

The key to effective flat roof repair is to recognize in upfront the areas that are likely to be susceptible to leaks. The best test alas is to carry out the work to close suspect gaps and then wait to see if the leak persists. So flat roof repair is heavily dependent on the integrity and intelligence of the contractor you hire.