Tonyh is right regarding high static heads and VFD. You can check the pump's operating point and efficiency at different speeds by plotting pump curves at different speeds (using affinity rules) together with the system characteristic. Or use software to create the plot.
You could try a wire mesh instrument. Rossendorf research institute in Germany (Dr. Prasser) has experience in developing wire mesh two-phase flow meters. A 20" version could be calibrated at Delft Hydraulics Labs over a wide range of void fractions.
Tremolo's approach may lead to over-conservative reverse velocities, peak pressures and finally a more expensive design. The best (only) way to model check valve behaviour correctly is to use the dynamic characteristic of the check valve (reverse velocity versus fluid deceleration).
The...
