If a system can support two (or more) waves where the frequency and wavenumber of one are twice those of the other, then the waves resonate. In non-conservative systems this can enhance the energy transfer between the wave and its medium, and leads to types of nonlinear interaction not found for non-resonant waves. In this paper a method is presented for locating resonant conditions. The method is particularly well suited to cases where there are several branches of the dispersion relationship and when the dispersion relation is computationally expensive to calculate, e.g. shear layers at high Reynolds numbers. It has also been tested on capillary-gravity waves where it is shown to give accurate results not only for the 2:1 resonance but also n:1 resonances where n can become quite large.