The photodarkening effect is a crucial factor that influences the stability, reliability, and lifetime of the high-power Yb-doped fiber lasers. The accurate physical characterization and diagnosis of the photodarkening performance of the Yb-doped fibers are the premise and basis for understanding the photodarkening mechanisms and for establishing the photodarkening elimination technologies. This study introduces the main physical parameters of the photodarkening performance. Further, the physical diagnosis methods used to assess the photodarkening performance are also reviewed. Additionally, the advantages, disadvantages, and application scopes of these methods are analyzed. The major factors influencing the photodarkening performance are subsequently discussed. Finally, the research development trend of the physical diagnosis methods is briefly discussed with respect to the photodarkening performance of the Yb-doped fibers.