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You should be able to tell at a glance - lath & plaster is old and can
look a bit uneven and may have wandering cracks, plaster board looks
flat and any cracks or visible uneveness will be along the board edges
in straight lines. If you shine a torch holding it up to the ceiling
and pointing across the surface it may show up the difference.
But if perfect you might not be able to tell so poke a hole where the
damage is and see if you can find any laths i.e. thin bits of wood, or
edge of plaster board with paper or card on both faces. (+ plaster skim
on the face side)

chees
Is there a light fitting you could unscrew? You might be able to tell from
that.

D