Usual way, is to try to screw into the joist timber, more secure that using plugs into plasterboard....
You can work out where the joists are located, using a strong magnet, to find the screw heads, holding the plasterboard, under the plaster. Screw heads, indicate where the joists are. Ceiling roses, have the two fixings off-set, so it's easy to get both screws into the timber joist.
Sometimes, you can poke a thin screwdriver, through the same hole the cables drop through, and feel where the joist is.