I have a fuel polishing system with a racor filter. I have no real way of telling how well the fuel is flowing out of the filter system. A piece of clear fuel line would do it but I don't think there is such a thing.
I would look for a visual flow indicator that is designed to work with fuel. Its nothing more than a little paddle wheel that turns inside a sight glass. There are others out there. I found this by doing a Google search.