A method for determining crosstalk coupling between a plurality of transmission lines in digital data transmission systems, in particular DSL networks. Firstly, at least one test signal (TS) is generated having a unique identification code (UIC) in the frequency domain (f), said code (UIC) being associated with a first transmission line. Then, the test signal (TS) is sent on the first transmission line. At least a second transmission line is monitored for detecting a crosstalk signal. It is determined whether the crosstalk signal is caused by the test signal (TS) of the first transmission line by means of a signature of the unique identification code (UIC) comprised in the crosstalk signal. In this way crosstalk between individual transmission lines can be determined without having to have recourse to conservative worst case assumptions. The method can adapt dynamically to a changing user/transmission environment, while obtaining unique crosstalk determination results at all times.