Amp output levels

Is there a standard way to check left and right output power or level of a stereo amp? My meters seem to be uneven when feed a mono signal and I'm wondering if it's the output levels themselves or the meters.
Amandarae...Your way will involve comparing measurements of two relatively large voltages. Even a small percentage measurement error of the large voltage will imply a large difference.

If you measure the difference (across the two hot amp output terminals) the voltage will be small (ideally zero) and measurement error will not be significant. (You will be on a low scale of your meter). If you want to express the difference in terms of dB relative to the output signal you will need to also make a measurement of the output (either channel).

By the way, an Oscilloscope is a useful tool, but not suitable for accurate voltage measurements, unless, perhaps, it is a digital scope that has a numeric display of peak voltage (in which case it is really a meter, not a scope).
Post removed 

Thanks for the explanation.

The reason why I mentioned the power resistor as dummy load is to simulate a loading condition and prevent any mishap to the speakers. I also mentioned an oscilloscope and a power meter ( I have HP power meter and Tektronix TDS 3052B 400 Mhz Digital scope)so that you can measure the voltage or power across the dummy load when the load is "hot". At resistor rating of 25 watts on the output of the amp, using a testone of say 2 dBm (796 mVp-p/1.58 watts), the resistance should not shift and thus accurate voltage can be measured across it. In fact, you can easily see and measure the sum and difference of the L and R signal with just a push of a button on the scope and plot the signal on top of each other.

For the record, I am not the one who mentioned AC Voltmeter. It is the other poster, not me.

In this particular case, I do not know if there is more accurate than mathematics. I guess we have our own little way of measuring electronic parameters.

Best regards,