Tell me about it. I've fixed my fair share of leaky roofs and sometimes you just make an educated guess and hope. I hate getting up into the ceiling but sometimes it's the only way to trace the water from the end point to the source. With tiled roofs however, it can be nice and obvious with something like a cracked tile or a tile that's shifted out of place.

^ This! Worked at a place that had a roof leak, there was a pipe in the false ceiling and the water leaked onto the pipe, then ran the length of the pipe dropping off at random points.

So we had this line of brown spots on the ceiling tiles, it looked like the whole roof was leaking, but it was just one leak following the pipe.