OK, in my experience field testing even under ideal conditions is very arbitrary and inconclusive. At the moment you know the discharge pressure only, even knowing the inlet pressure could give you some idea of the total head, but to what degree of accuracy.
You can measure the input power to the motor, and with a lot of if buts and maybe's assume the motor efficiency, from here you can guess the pump input power and look at your pump curve - is this a test curve or a generic published curve for this particular model pump - now, is the pump in the same mechanical state on which it was tested (assuming it was) running at the same speed etc.
Lots of unknowns and guesses as a field test can never reproduce a test result and if a generic performance curve the field test could be near useless - of course this depends on what accuracy you want.
I have experienced a number of pumps rejected by site tests as not performing as quoted / sold etc. but on return to the manufacturer and retested under test conditions they perform as stipulated.
The only way to establish accurate flow is to measure it.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)