Hollow walls and ceilings may be surfaced with lath and plaster (pre-1920s houses) or plasterboard.
Identify which type you have by drilling a test hole and examining the drill dust. (Use a HSS drill bit for hollow walls):
Grey sandy dust (and possibly sawdust if you hit a lath) shows you have lath and plaster walls and ceilings.
Fine white dust and paper shreds on the drill bit shows you have plasterboard.
On external walls with hollow dry lining, drill on through the plaster or plasterboard lining until you hit solid masonry.
Withdraw the drill and measure off the cavity depth. Some hollow wall fixings will not work if the cavity is less than about 25mm deep.
