  Future large liquid-scintillator (LS) detectors are competitive with and
complementary to the water-Cherenkov detectors on the searches for diffuse
supernova neutrino background and nucleon decay. In a companion paper, we have
performed a systematic calculation of the neutral-current (NC) background
induced by atmospheric neutrino interactions on $^{12}{\rm C}$ nuclei in LS
detectors, which are expected to be crucially important for the experimental
searches for the diffuse supernova neutrino background and nucleon decay. In
this paper, we perform a systematic study on the measurement of the NC
background and evaluate the associated uncertainties. We first exploit the
characteristics of the NC background, in particular, the multiplicities of
neutrons and pions, and the possible association with unstable residual nuclei.
It turns out that the neutron multiplicity distribution is very powerful to
discriminate among different models. Then, we develop a maximum-likelihood
method to allow an {\it in situ} measurement of the NC interactions with a
triple-coincidence signature. Finally, a data-driven approach is proposed to
evaluate the uncertainty of the NC background in the search for the diffuse
supernova neutrino background. We conclude that future large LS experiments
like JUNO (Jiangmen Underground Neutrino Observatory) will be able to make a
unique contribution to the worldwide data set to improve the prediction of
atmospheric neutrino NC interactions on $^{12}$C.
