I want to hang a ceiling fan at my apartment, where the ceiling is made mostly of a sandy material. How do I find the concrete beam going through it, assuming I do not have access to the blueprints of the house?
Have you tried a stud-finder device? They are normally used with thin walls to find the wooden studs behind the walls. I think they are really ultrasound devices and they just look for the return time. They should work on any homogeneous material, as long as it's not too thick.
Have you tried a simple druminess test. It doesn't work in every case but simply tapping the ceiling with a lump hammer and listening to the sound generated can give you a good idea about the make up of the wall.