  In the context of Monte Carlo simulations, the analysis of the probability
distribution $P_L(m)$ of the order parameter $m$, as obtained in simulation
boxes of finite linear extension $L$, allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, $P_L(m)$ becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of $P_L(m)$ at
the peaks ($P_{L, max}$) and the value at the minimum in-between ($P_{L, min}$)
becomes $L$-independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead $\Delta F_L := \ln (P_{L, max} /
P_{L, min}) \propto L^\theta$ should be observed, where $\theta>0$ is the
"violation of hyperscaling" exponent. Since $\theta$ is substantially non-zero,
the scaling of $\Delta F_L$ with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, $\Delta F_L$ versus
$L$ was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.
