I'm not sure either of the above suggestions will "prove" the pump is OK.
3 other things have to be considered.
1. Head pressure of the coolant above the pump when installed
2. System pressure
In my earlier days i experienced good pumps that would go south while an engine was out being machined. I attributed this to "drying" out. I the started placing the pumps in a bucket of coolant and had no more failures. I stopped doing this years ago as pumps became cheaper and it was not worth risking a comeback on a customers car.
On a somewhat related incident, I recently had a real strange one. The car had come in for a coolant leak. Ok I pressurized the system and immediately noted several leaks at the hose clamps. Tightened the clamps and left it under pressure over night. Easy fix.
A week or so later the customer calls and says it's leaking again ? I have him check the clamps and they are all tight. he brings the car back in and mentions that the leak only appears after it's been sitting 2 or more days, STRANGE
I pressure test again with no problem noted. So I just leave it sit. Sure enough on the 3rd day the weep hole on the bottom of the pump starts very SLOWY dripping. Like 1 drip every 1/2 hour
In over 50 years i have never encountered this A new pump and all was well