A signal leakage in a cable network may be detected by using a test device to obtain a spectrum of an electromagnetic wave propagating in vicinity of the cable network, and automatically detecting QAM channels in the obtained spectrum by detecting characteristic spectral roll-offs at boundary frequencies between QAM channels of the cable network. A test device may be used to determine which QAM channels are currently active on the cable network, thereby facilitating automatic QAM signal leakage detection.