In days of old, with a difficult to find pressurisation leak (no visible tobacco stains or window condensation) we would do a ground pressurisation run, mostly it required an engine supply as the APU would only give about 0.5psi diff. a mechanic would go around outside feeling windows and doors for air escape.
The same could be done with a down route / on ramp ADC change to confirm, by watching rate of climb/decent indicator, whether the hose had been connected leak free, external ports were plugged for this!
I believe now, best safety practices and all that, almost a steel net is required over the aircraft, hence finding leaks nowadays must be quite a task?