I see this all the time and nine times out of ten its the grout or a bad pan under the shower. Before anything else I try to determine if the leak is constant or if it is periodic. If it is constant there is a good chance the leak is in the pressurized water lines. Usually the leak is periodic so I have a series of tests that I perform to track it down. Sometimes a quick visual inspection of the tile will show that the grout is shot and is the most likely cause of the leak but I will often complete the rest of my tests to be sure. What I do is first fill the tub half way and drain it. This will tell me if it's in the drain pipe. For a shower with a lead or vinyl pan I block the drain and fill the base with water. This will tell me if the pan leaks. Then I remove the shower head and put a 1/2" cap on the shower arm and turn on the pressure. This will tell me if there is a leak in the pipe between the shower valve and the shower arm. If no leak has shown up by then I tend to think the leak is water bleeding through the tile due to bad grouting or that water is escaping the shower and going down through flaws in the bathroom floor. I can check this by taping up a plastic dropcloth inside the shower covering all the tile work and having the customer use the shower normally for a day or two. If the leak has suddenly disappeared then we know it coming through the tile. A few cups of water on the floor will show a leak through bad tile or a cracked floor base.