If the leak is a major leak, then check the connection where the pipe comes out of the wall for the shower head. If it is a minor leak, then check all the tiles for openings in the grout, check around the base between the base and tiles, check along the floor, behind the faucet plate and at the drain port for any openings in the grout or tub sealant. These are the prime areas where water will travel while you are taking a bath. This water, once behind the wall, will make it's way downstairs by way of a path of least resistance.