The most reliable way to tell if there is any flow through small bore tubing is the use of a “show flow”. This is basically a device with a float / ball inside a glass chamber. When product flows through the show flow, the float / ball is lifted and bobs inside the glass.
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.