A critical issue in optical chaos-based communications is the possibility to identify the parameters of the chaotic emitter, and hence to break the security. In this paper, we study theoretically the identification of a chaotic emitter that consists of a semiconductor laser with an optical feedback. The identification of a critical security parameter, the external-cavity-round trip time (the time delay in the laser dynamics), is performed using both the auto-correlation function and delayed mutual information methods applied to the chaotic time-series. The influence on the time-delay identification of the experimentally tunable parameters, the feedback rate, the pumping current and the time-delay value, is carefully studied. We show that difficult time-delay-identification scenarios strongly depend on the timescales of the system dynamics as it undergoes a route to chaos, in particular on how close is the relaxation oscillation period from the external-cavity-roundtrip time.