How can i measure the power output of a audio amplifier?

 

You can always work from the power supply, and calculate the maximum current possibel times the voltage, and say that it is capable of that power out.

Otherwise, you need a load resistor that can handle the expected power and attach it in place of the speaker. Apply a 1 KHz sine wave signal at the amplifier input and increase it until the sine waveform across the load resistor starts to clip. Back off until the sine wave is still undistorted, and measure the voltage and current being developed across the resistor.

Note that the resistor value can be quite important. Speakers used to be characterized as 8 ohms. To avoid relatively large voltages driving current, speaker impedances are sometimes down around 2 ohms. You need to know the expected load impedance in order to select the correct load resistor.

 

For your own information when you do measure the Vac at the output load.
The formula is = VxV/R
Where P is wattage, V is output voltage (RMS) & R is the load resistance on the output.

 

Is it okay to take the reading across the speaker using a analogue multimeter

 

An analog voltmeter is made to measure the 50Hz or 60Hz mains frequency. Its accuracy at 1kHz to 20kHz will be poor and it will probably measure too low. You need a good speaker and good hearing to hear when the amplifier begins to clip so you can reduce its input a little. I use my oscilloscope.