A measurement procedure is presented for the determination of mode splitting (or mode sorting) in asymmetric Y-junctions. This procedure avoids measurement errors introduced by bends, tapers, small damages in the waveguides outside the coupling region, variation in fiber-chip coupling, etc. It is based on the analysis of the normalized coupled power in an optical coupler in which a mode splitter is used. The method is illustrated with a mode splitter on lithium niobate. A value of 29.2+or-0.5 dB for the suppression of the unwanted mode is found, showing the applicability of the procedure.