If the water has been leaking for more then a few days, you have mold there. It takes around 48 hours for mold to begin to develop. The question is was the leak stopped? If so, and the tiles are secure and there is no stains coming through the wall, then the amount of mold present most likely is not enough to raise any real concern. However if you do see stains that has resulted from the leaks, or the area is prone to moisture then you need to open the walls and remove even the slightest amount of mold that has developed or it will continue to grow until it becomes a real health hazard.
There are Boroscopes that are little cameras on the end of a flexible tube that can be inserted in a small hole to look and see if mold has begun. There are also tests using a wall check system that draws air out from behind the wall surface and impacts any mold spores that may be present where you cannot see them. But those methods are better left to your local mold testing company.