As the Chemists can tell you, testing protocols & systems will never keep up with the ability of fuel blenders to blend rocket fuels. Lost cause.

Only solutions are "tagged" fuel with a known % of chemical tracer (as at the RunOffs the last few years), or else do what the go-karters do - a "pump around", where everybody's fuel is mixed in one big tub, and everyone has to draw fuel off that tub.