Extend Unterminated Line (EUL) structure allows crosstalk measurements with perfect termination and halves the number of test ports required by the traditional method. Therefore, EUL is a time-efficient and convenient structure for a test vehicle design of high-speed interconnects. In this paper, the potential errors in the EUL measurement are analyzed. In addition, an error detection method to check for causality, passivity and reciprocity is demonstrated. It is the first time to propose the best practices and robustness-enhancement techniques for EUL crosstalk characterization.