As recommended by a plumber, we tested a bathtub for leaks before taking it inside.
The drainage pipe was fixed above the level of water overflow (on the left secured with a piece of wood) and the tub was filled with water above the overflow opening. It was checked after 20-30 minutes. The pipe going down from the overflow opening was wet and water was slowly dripping down. This is a dangerous problem because in most cases it could be a small leak and water remains unnoticed under the bathtub when it is installed