Use a sine wave as you said, start very slowly increasing the amplitude, but use a microphone to determine the point where the sine wave starts to distort past your cutoff point. It should start to distort past 1% well before the speaker blows, then you know the 'practical' wattage of the speaker. Audiophiles use absurdly low distortion values. But 1% is roughly the limit of where a human will notice the distortion occuring, depending on how well the speaker produces the harmonics involved.