Cat fines will not block purifiers. They will likely first result in damage to fuel pumps and piston rings liners.
The test method in a laboratory is to ash the sample and then create a solution which is injected into a atomic absorbtion (AA)tester.
There is no cheap tester for ship's use. You...
as long as you keep the fuel above its pour point temperature by about 10C it will remain fluid. When you need to transfer refer to the viscosity of the fuel. You need to have a maximum viscosity of about 600cSt to allow pumping and good flow.
good luck