As a key component of the proton exchange membrane water electrolyzer (PEMWE), the porous transport layer (PTL) not only provides mechanical support but also facilitates the supply of reactants to the electrode and the removal of produced gases and ensures efficient electrical and thermal management. Commercially available PTLs are often repurposed for other applications, such as filtration, and are not specifically tailored for PEMWE applications. Given this context, research output on PTL development has increased notably in recent years. Optimized, structured PTLs with preferred properties require applicable, relevant, and convenient diagnostic tools for PTL material development. As such, this work aims to identify and review a wide range of techniques for evaluating developed PTLs, including electrochemical techniques, custom-engineered cells, operando diagnosis, ex situ characterization, and postmortem analysis. By providing detailed information on these characterization techniques, this review aims to catalyze further research and development in the academic and industrial sectors, enhancing the understanding, development, and quality control of PTL components.