The most reliable way to tell if there is any flow through small bore tubing is the use of a “show flow”.
Liquip's “Show-Flow” visual indicator provides a observable status of flow in the fuel system by the yellow paddle-wheel spinning.
Some of these devices are also equipped with a check valve. When installing these devices, the arrow on the casting needs to be adhered to. The show flow is most commonly used together with an air eliminator. It is also sometimes fitted to the thermal relief.