The pump in many pressure washers are positive displacement pumps and as a result the outflow of the pump is closely connected to the RPM, strokes, etc, as long as the fluid is relatively non-compressible.
Just to back up a bit - take an air compressor with a piston in it that goes up and down. Air is let in then pushed out. The compressibility of air allows for a wide variation in the mass flow rate so it is difficult to relate the strokes per minute to some standard volumetric displacement. Many high pressure water pumps use pistons, like the air compressor. Because water isn't compressible like air, when the piston strokes a fix volume of water is moved. Yes, there is some minor leakage but it's just that - minor.
If this is the case you might monitor motor RPM or pump strokes as a way of determine flow rate. An orifice could be used with a pressure drop method but I don't think the accuracy will be improved over the method I described.