If the ceiling is wet, lets start with the fact that the drywall needs to come out. If the leak magically went away, and the ceiling wasn't too wet, you could try to get enough coats of stain killer on there to cover, and hope no mold is growing inside. SO, let's just forget that part, and taking down the drywall is maybe the quickest way to find where the water is coming from. You mentioned toilet or shower, and that is a broad area. In the shower, we have the hot and cold, and leaks in the valves could possibly be fixed without tearing out the ceiling, if you do the detective work by removing the trim for inspection. If the drain is leaking, that will likely need to be fixed from below. If the toilet is leaking such as the wax ring, you can fix it by pulling the toilet and replacing the ring. But the quickest way to see what's leaking is from below.SO. There is little downside to opening up the ceiling. Drywall repair is fairly easy and inexpensive. The view may be up inside may be priceless in terms of locating the leak and possibly finding a way to fix this yourself.