If you just want to know whether or not the fluid is flowing, or want to confirm flow rates rougly, this low-cost and compact sight flow would the best solution for such needs.
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Flow control valve
Flow controller
Manifold
Vortex flow meter
Turbine / Paddle wheel flow meter
Differential pressure flow meter
Bypass flow meter
Variable-area flow meter
Flow switch
Sight flow
Electromagnetic flow meter
Coriolis flow meter