plasterboard fixings

Hi guys, Putting together some IKEA PAX and at the step of fixing the top of it to the wall. The wall in question is a dot and dab wall and I am wondering the best fixing to use. I know corefix is used widely for heavy hanging (kitchen cabinets etc.) but I also know that when fixing wardrobes...

I want to secure some brackets to an internal wall hold some heavy things. I'm looking to get some strong fixings for this (I know which ones I want to get How do you work out whether it is dot & dab or studwork (with cavities) behind plasterboard on the internal side of an external wall?

Ive seen snap toggles that claim upto 108kg on 12.5 mm thick plasterbord, so theoretically if I use 2 , plus a couple of extra smaller fixings that should be fine ? Its a water heater Im trying to fix. Ideally dont want to add wood as the fire risk assessor isnt keen on it. The other idea I...