The invention relates to a method of checking the correct operation of a signal transformation wherein a input signal is transformed into an output signal. The method comprises: deriving a first robust feature from the input signal; deriving a second robust feature from the output signal; comparing said first and second robust features; in case of sufficient sumilarity, concluding a correct operation of said signal transformation, and in case of insufficient sumilarity, concluding a false operation of said signal transformation. In a special embodiment, the method is applied wherein the first robust feature is embedded in the input signal through watermark technology, the thus obtained signal being transmitted to a receiver so as to retrieve an output signal corresponding to said input signal.