To make sure that you have enough pressure to overcome the relief valve either disconnect return again and check for flow. Or close off the return and power shouild be greater with a smoother idle.

You will not hurt anything with the test. The power with the return line closed off may suprise you if the lift pump is putting out what it should.

Really the base pressure in the injection pump should be checked with a gauge. The operating pressure is more important than people think and the relief valve spring can weaken with time lowering it. Yes the car will still run well but the base should have the recommended pressure or as high as 19 pounds. Certainly more than ten pounds.