hi lilacben plasterboard is a reasonably strong material to support from, depending on the metal/wood/dot&dab that is holding it up and this is where you could do with finding some supports under it, possibly by drilling some very small test holes to locate them or rely on what jack knoff has suggested.
but the likelihood of finding two supports at the correct spacing is remote.
also i would,nt hesitate to cut out a section of plasterboard and install the supports in the correct spacing and then reboard, given the value of what you,re supporting as against £5 of plasterboard and a £7 bag of multifinish, but then some people might not like to go to that extent.
when tiling skimmed plasterboard is reckoned to support 20 kg per metre square inc. adhesive and grout, if the plasterboard is correctly fixed to its subtrate or support.however thats a distributed load of fairly small, thin tiles and your TV is on a pair of brackets on more of two point loads, i assume.for peace of mind i would want to make certain its securely supported
sorry for such a long reply, hope it helps