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.

Good speakers have a detailed datasheet complete with plans for a recommended enclosure. Garbage speakers don't. I guess your speaker is ....

The point at which you can 'see' the distortion on a scope is just about the point you want to stop for ears as well. Like audiguru said though, you're going to find out just how crap those crap speakers you have are =) Mind you they need to be tested in the enclosure you're going to use.

A more basic problem is that speaker 'wattage rating' is a pretty loosy goosy measurement that is rarely accompanied with the necessary parameters needed to understand it's meaning.

While consumer stereo amplifiers at one time required giving the parameters of their output power ratings (60s era FTC regulations) I've never seen many speaker manufactures qualifying their speaker 'power ratings' let alone stating any measurement standards. At best you get sometime like maximum recommend power rating, nothing about at what frequency, at what wave shape, at what duration, at what distortion value. It's a crap shot really.

Lefty