How to measure the power output?

This is a very dirty but within 10 percent method. Get a 100 watt light bulb. Set your multi-meter to measure amps. Run your amps power up till you hear clipping on existing speakers. Hook up the bulb and meter in parallel with your amp output. Run the volume to the power level you first began hearing the distortion. Now read the amps off the meter and write it down. Now measure the resistance of the light bulb. The wattage is the amps reading squared times the resistance of the bulb. This is a safe test because if you run too much current the bulb will burn out opening the circuit with no harm done other then a bad bulb. You can use two bulbs is parallel also. The reviewers us a oil bath 4 or 8 ohm resister rated at 500 watts etc.

Lamp filament used as dummy load is not a valid measurement, because filament resistance varies with conduction/temperature. You need an audio oscillator, dummy load power resistor & oscilloscope, & also need to know ohm's law (I'm not going to try to teach this here). Take it to a shop, which can do this for you accurately for a few bucks, or probably free if you're a good customer.

Materials needed: music source, amp to be tested for power output, speaker w/power handling greater than amp's output, AC meter. Hook up speaker to amp & simultaneously connect AC meter to amp's speaker output. Play music on amp and turn up volume slowly just till distortion is audible. Peak voltage over 2.83 (volts per watt) = aproximate wattage output.