It is never a given that any part of the controller output is grounded. That can make resistance-to-ground measurements superfluous, unless all conductors are tested.
As noted, "I got 24 volts" says nothing about the actual circuit path quality.
Even resistance measurements may turn up nothing. Old-time mechanical controllers gave zero info about electrical faults. This is where the clamparound ammeter came in, as it could show a higher-than-usual current draw on a solenoid that refused to go to shorted status while the repairman was watching. They had to have voltage applied to show the fault.
The yellow clamparound ammeter was a Sperry model that had an extra range of 1.5 amps full-scale, good enough for the higher current draws of old-time solenoids.