I just came up with a much better answer, but couldn't edit my original one. Here it is:

There are four sources of the drip:
1. Leaking drain system
Test for this by running a hose from outside and pumping water directly into the shower drain. Try not to let water pool in the floor of the shower or around the edge of the drain. Do this for as long as it takes to produce the leak or convince you that it's not the source. If this produces the leak, you're going to want to open up the floor or the ceiling below. The ceiling is almost certainly easier.
2. Leaking shower floor, cracks in tiles or grout, infiltration from places where water splashes during a shower. Use the hose to splash water on the floor of the shower. Then try anywhere else that water might touch during a shower.
3. Leaking shower fixture in the wall
If neither of the above produce the leak, run a lukewarm shower, again running it long enough to let you conclusively say if it causes the drip. If it does, there is a leak in the connection between the shower control valves (the knobs, or the single valve if it is the modern kind of shower) and the shower head, or in the valves themselves. If this the source of the leak, you might be able to see exactly where it is leaking by taking off the handles and shower head and removing any chrome trim that covers the holes where this stuff sticks out of the wall. You might get lucky this way, but you might have open up the wall to find the leak.
4. Condensation from long, hot showers
If none of the above reproduces the leak, close the windows and doors and run an extra hot shower. You could have water condensing inside the bathroom, then running down the walls and getting into the floor.

Looking at option 2, I would try to regrout any apparant cracks that you have right now. Second, look for unexpected ways that water might be leaking...like, when the shower diverter is left in the on-position or in half-way and the water then doesn't have a path to get out, but is backed up. Also, we lost a soap dish once after it was discovered that my wife was proping her leg upon on the tile soap dish to shave her leg,and the added pressure cracked the tite grout. Second, water running down her leg then got to right where the tile was cracked. Another bad spot, water that flows out from around the shower curtain and pools next to the tub, penetrating the floor and rotting the subflooring.