A DMT (Discrete Multi-Tone) test method is employed by a single test device to estimate the theoretical and practical data rates of a cable under test for a pair of target ADSL (Asymmetric Digital Subscriber Line) DMT modems. The DMT test can also be used to analyze the cause of a problem, or to predict a problem, whereby a pair of ADSL DMT modems could fail to synchronize. The method includes (a) measuring the frequency characteristics of the cable with discrete tones; (b) measuring the cable noise and interference for every sub-channel of the whole bandwidth; (c) determining the theoretical and practical data rates for every sub-channel, based on the measured frequency characteristics and noise characteristics, as well as modem parameters selected by the user; and (d) estimating the theoretical and practical data rates for the ADSL bandwidth based on the ADSL DMT standard selected by the user.