You can install a hyd gauge in a tee in the IN port of the first valve and check the pressure for any valve downstream.
To check flow, hyd shop have a flow gauge and pressure setup with a needle valve, relief valve and they can check the pressure and flow.
You can check the GPM by timing the flow in a bucket.
A 6 GPM pump will pump 6 gal per minute, so to verify this, put the output hose in a 5 gal bucket and see if you can get 1 gal in 10 sec.
A 12 GPM pump should flow 2 gal of fluid in 10 sec.
Max rpm produces max flow.
Your 10 GPM pump = 1 gal in 6 sec.
10 gal in 60 sec