Evaluation of the molting stage of crustaceans is essential for studying their mating behavior because of molt-dependent changes in their behavior. For example, in species in which females mate just after molting, sexually mature premolt females are ready for courtship and postmolt females are ready for copulation. Non-invasive methods of evaluating molt stage are essential in studies of molt-dependent mating behavior so that normal behavior is not disrupted by the evaluation itself. The goal of the present study was to establish a non-invasive evaluation method for the helmet crab Telmessus cheiragonus, which is a model species for research on crustacean sex pheromones. We have identified morphological and behavioral indicators of molt stage. A morphological indicator of molt stage was a change in the color of the edge of telson and sixth abdominal segment, from a dark line to red in premolt animals to white in postmolt animals, which can be observed with the naked eye. A behavioral indicator of molt stage was that premolt females tended to fold their walking legs and chelae to adopt a precopulatory guarded position during artificial handling, whereas late postmolt stage females with developed ovaries and early premolt females in an early molting stage did not show this behavior. These non-invasive morphological and behavioral characteristics are useful indicators of the premolt stage of sexually mature female helmet crabs.