How to find type of wall separating flats in same building
Concrete block wall and dot and dab plasterboard would be pretty standard between two dwellings because of fire regs. 

Grab some post-it notes and stick them on the wall where it sounds solid. See if you discern a pattern, i.e rows of 'dots' (or 'dabs', I never did figure out the difference!). It will (or should be) solid along the bottom and the top (a continuous dot or dab).

Make sure your bracket sits on solid areas or you'll crush the plasterboard into the 10mm (or so) gap behind the boards.

If you have to go where it's hollow, drill an oversized hole and pump/push plasterboard adhesive into each hole forcing it in to spread it out behind the board as much as you can. We push the plastic disk out of an empty caulk tube with a long dowel etc., fill it up with board adhesive and pop a fresh nozzle on and you're good to go. Pump away! 

Wait until it's dried for a day or three and then drill your holes into the concrete block. 

Make sure your fastener is wholly holding in the block, i.e. not the plasterboard or its adhesive.