Fluid transport in low-temperature water electrolyzers (LTWEs) and low-temperature fuel cells (LTFCs), spanning multiscale heterogeneous structures, significantly influences the production rate and energy conversion efficiency of green hydrogen energy. However, the contributions of underlying transport processes to the practical performance remain difficult to interpret, leading to a largely black-box understanding of transport-performance relations. This challenge is fundamentally constrained by the capabilities of available analytical techniques, particularly under realistic operating conditions. To address this gap, this review establishes a measurement-centred framework that links fluid transport phenomena with their intrinsic observability, thereby defining the current limits of performance interpretation. We first derive the key requirements of analytical techniques from fluid transport-induced performance limitations. Based on these requirements, we critically assess the capabilities and limitations of established optical, X-ray, and neutron based analytical methods. Particular emphasis is placed on emerging strategies, including embedded miniaturized sensors and ultrasonic techniques, which enable the transition from lab-scale observations towards operando diagnostics at device and system levels. Rather than treating these analytical techniques in isolation, this review highlights their complementary roles, defining the current observable space of fluid transport across scales. Recognizing the remaining gaps, it suggests that synergetic combinations of complementary analytical techniques may improve the effective interpretation of transport-performance relations across scales.