How to fix new light with glass shade to plasterboard ceiling?

I have 3 x CFL ceiling lights on a new build flat (leasehold) but I dont like the colour temperature (4000K they provide). Looking to replace with incandescent bulbs with glass shade circa 1KG in weight. The ceiling is plasterboard - before I employ an electrician I need to understand if theres a ceiling joist they can screw into Ive heard the best way is to use a powerful magnet to search for the joists as the electronic ones are unreliable. Ive tried knocking on the ceiling but cannot hear a noticeable change in resonance. Ive also read on forums the ceiling may be 'false', be metal RSJs above or potentially concrete? Advice would be appreciated