Look at the wall area surrounding any fixtures and fittings, such as a sink or bathtub, whose pipes run into the wall.
Slide a stud finder that also detects metal along the wall area running vertically and horizontally away from where the fixture's pipes disappear through the wall.
Identify any wet spots on the wall, to detect the location of a leaking water pipe.
Go outside and locate pipes extending up through the roof. Back inside, look at the wall area directly beneath these pipes as the locations for drain pipes.