Elininate the easy things first: take the trim off of the shower control valve and then turn it on to look and see if there's any water behind the wall coming from the valve. Do the same thing at the shower arm. If it is there, you'd normally see it dripping down past the valve once you have the trim off.
Are there any niches or benches in the shower? If there are, take a bucket and pour some water in there and see if that isolates the leak.
But, first, you could take the drain cover off, if you don't have a drain plug, you can use a rubber balloon, and inflate it inside of the drain. Then, fill the pan up to just below the curb and let it sit. See if it leaks out from there. Let the water sit overnight to see if the level drops and you notice if it is leaking visually. If you do this without running any water in the shower, and it leaks, you know it is the pan rather than a leak somewhere in the walls or from the supply. It can be tricky blowing up a balloon at the floor while standing on your head! They do make plugs specifically designed for this, but a balloon is cheap and will work if you get it in far enough.
Unfortuneately, there are lots of showers built incorrectly. If you're lucky, it may just be in the valve behind the wall, and you might be able to fix that without tearing up much of anything. If you're really lucky, it is that and you can access that wall from the other, non-tiled surface which is generally a lot easier to repair than the tile itself.