About the only way to test the pump is with a flow meter equipped with a load valve, a flow meter alone will not do the job. I say this because the pump may be pumping the required GPM at a low pressure but as the pressure rises flow bypassing in the pump, etc will cause the flow to drop off. I have tested excavator pumps that appeared good until I loaded them and at that point the flow slowly went away to nothing, or nearly so.
That said, as bad as I hate to tell you this, there is no way for you to test the pump yourself without spending a fair amount of money on a flow meter with an integral load valve, or a simple, inline meter with a needle valve to use for loading. Either will work but the ones with the integral load valve typically also have a rupture disc as a safety feature in case the load valve is closed too far. I have had pumps that were sticking and as I shut down the load valve they would suddenly unstick and increase the flow more than the meter could handle valved off thar far, and they would rupture the disc to allow the excess flow pass. Like I said, using the inline ones, with a needle valve inline downstream of the meterto load the system will work but it's just not as safe when used on something like an excavator pump that has a variable/pressure compensated flow.
As someone who does this kind of thing for a living, ultimately the best advice I can give you is to call someone who knows what their doing, and has the right equipment to do the job. Good luck.