A method of determining at a receiver whether a received signal comprises a pure tone signal component. The method comprises: measuring a received signal over a measurement period; calculating, using maximum likelihood hypothesis testing, a likelihood ratio value for the measured signal and, determining, based on said likelihood ratio value, whether the measured signal comprises a pure tone signal component. The likelihood ratio value is a value indicative of the ratio of a likelihood LFSC that the measured signal comprises a pure tone signal component, and a likelihood LnoFSC that the measured signal does not comprise the pure tone signal component.