Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
If you run a pure tone out of the amplifier (use signal generator on input) and you know the impedance of the speakers at the frequency of the test you might get an approximation of the amplifier power by measuring the rms voltage across the speaker terminals - and dividing the voltage squared by the impedance to get rms watts. If do the measurements with music as the input then you'll get a constantly varying measurement - that is unless you have a measuring instrument that can deal with that. I recall that CDs with test tones could be purchased - I was going to purchase one once but never got around to it (though my friends have offered me the use of their round tuits).
A better way might be to use an appropriately sized power resistor in place of the speakers so you remove the reactance from the measurement process.
Note that industry standards probably exist that do a better job qualifying the conditions, test fixture, etc - all of which can impact the precise measurements.
FYI - watts= (voltage times voltage) divided by resistance
Above assumes your voltmeter gives rms readings.
More math - if a sine wave then peak voltage = rms voltage times 1.414
More math - if a sine wave then rms voltage = peak voltage times .707