How can one identify drainage pipes under plaster?

With what do you knock? And can the furniture assemblers do something like that?

With the hand or carefully with something hard. A wooden spoon, a paintbrush handle, a hand broom handle, a rubber hammer, a screwdriver handle...

Hello,

I would also expect that the position of the downpipe could be determined by tapping on the wall.

If not:
There is a very good photo where, using a rule of three with the known distance of, for example, the left sockets in reality, the central distance to the downpipe from the starting point could be determined:



Here, as an example, assuming a distance of 4 m between the two left sockets in reality and a measured distance on the photo of 22 cm, a central distance to the downpipe from the left socket would result in 2.18 m. Due to perspective, there may be deviations, but it should be sufficiently accurate.

Best regards

Guido

Thank you very much for the helpful tips. Our installer has determined the center of the drain pipe and attached a piece of tape to the floor there. The wardrobe is now hanging/standing. The tape will remain there stuck under the wardrobe forever for all following generations.