You would have to know that the performance of the pump matched the pump curve. Since manufacturers may have different practices about margin of performance on pump curves, and wear on an 18 year old pump is unknown, I would not rely on the curve and flow measurements to determine pressure at some point in the system.
I would find a way to get into the system and measure pressures directly with a gauge.
A 5GS07 is a very high pressure pump for a geothermal heat pump. My curves show that it delivers 5 GPM at about 370 ft of head (160 psi). I would expect that pressure drop across the water side of an evaporator would be on the order of 10 to 15 psi. I wouldn't rely on the accuracy of trying to infer the pressure drop in a heat exchanger for those conditions.