Massive protostellar discs are the sibling circumstellar structures of protoplanetary accretion discs. They form, evolve as a scaled-up version of the surroundings of low-mass stars and both formation mechanisms are unified within the so-called burst mode of star formation. This picture naturally links the development of gravitational instabilities in centrifugally balanced accretion discs to the formation of gaseous clumps and stellar companions which will influence the future evolution of massive protostars in the Hertzsprung–Russell diagram. We perform molecular line emission plus dust continuum radiative transfer calculations and compute synthetic images of disc structures modelled by the gravito-radiation-hydrodynamics simulation of a forming stars, in order to investigate the Atacama Large Millimeter/submillimeter Array (ALMA) observability of circumstellar gaseous clumps and forming multiple systems. We show that substructures are observable regardless of their viewing geometry or can be inferred in the case of an edge-viewed disc. The observation probability of the clumps increases with the gradually increasing efficiency of gravitational instability at work as the disc evolves. Our results motivate further observational campaigns devoted to massive accretion discs as around the protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a probable signature of disc fragmentation and accretion variability.