  The Atacama Large Millimeter/submillimeter Array (ALMA) revolutionised our
understanding of protoplanetary discs. However, the available data have not
given conclusive answers yet on the underlying disc evolution mechanisms
(viscosity or MHD winds). Improving upon the current results, mostly based on
the analysis of disc sizes, is difficult because larger, deeper and higher
angular resolution surveys would be required, which could be prohibitive even
for ALMA. In this Letter, we introduce an alternative method to study disc
evolution based on $^{12}$CO fluxes. In fact, fluxes can be readily collected
using less time-consuming, lower resolution observations, while tracing the
same disc physico-chemical processes as sizes: assuming that $^{12}$CO is
optically thick, fluxes scale with the disc surface area. We developed a
semi-analytical model to compute $^{12}$CO fluxes and benchmarked it against
the results of DALI thermochemical models, recovering an agreement within a
factor of three. As a proof of concept we compared our models with Lupus and
Upper Sco data, taking advantage of the increased samples, by a factor 1.3
(Lupus) and 3.6 (Upper Sco), when studying fluxes instead of sizes. Models and
data agree well only if CO depletion is considered. However, the uncertainties
on the initial conditions limited our interpretation of the observations. Our
new method can be used to design future ad hoc observational strategies to
collect better data and give conclusive answers on disc evolution.
