How to check the power output of home amplifier speaker terminals?
Audio amplifiers basically produce a voltage output. The current out depends on the speaker connected. Lower impedance speakers will make for more power out. 

Usually, you can connect speakers in parallel to the amp outputs. This will draw more current from the amp, and will increase distortion some.  If you don’t overload the amp, no harm will come to it. 

Also, different  speakers will have vastly different volumes for the same power input. 

With a voltmeter, you can see the voltage across the speaker. With a second meter, measuring current, you could determine the current and so compute the power. If you know for sure the impedance of the speaker, you can compute the power from the voltage, but real speakers have a varying impedance with frequency. If you replace the speaker with a power resistor, then you can compute the power output from just the voltage, since the resistor impedance is constant and known.