Two words - Binary Search. Or split in half to search one half at a time.
Apply water to the outside to one half of the area that could be leaking
. No water inside than dry off a little with a towel and apply water to the other half of the area that could be leaking.
When you find which half is leaking, dry off (inside and out) then divide the leaking area in half
and repeat process in that area.
Continue until area where application of water leaks
to the inside is a small enough half to be isolated to one item or location
I like a watering can or shower type head on the hose with pretty low pressure. Need to control the area that is getting water applied.
Bear in mind water can flow sideways along an edge or seam before continuing on to the floor. I have used light
dusting of baby powder to make leak show up. Any moisture on baby powder turns it dark. Sprinkled along edge of floor and wall it helps show where water first shows up at floor even if its just a little seep right down at the edge.
First step to finding a needle in a haystack is divide the haystack in half. :-)