Power is voltage squared divided by resistance (the load); Paramount wired for 8 ohms would produce 8 volts RMS at full power with a sine wave. A true power meter would have to measure both voltage and current and compute the product - pretty difficult!
I would recommend a VU meter (look up the wikipedia article). They are not cheap, but they are an audio standard. They reflect the perceived loudness pretty well, and the averaging time makes them about as fast as the human eye can follow. (There are of course many other meters that measure different things - peak voltage, RMS voltage, average voltage, etc, and with different averaging times.)
A standard VU meter would read zero VU at 1.21 volts, which is 183mW at 8 ohms. The full 8 watts is 16dB greater. But on well recorded material with good dynamic range, the peaks will be around 14dB greater than the VU indication, so with a VU meter on such material, the amp will be close to clipping on peaks when the indication is above +2VU. On compressed material, or when overdriving the amp (most of us do that more than we think!) a standard VU meter will be pegged, so an attenuator ahead of the meter would be a useful modification in many systems. Note that for standardized ballistics the meter should be fed from a constant 3900 ohm resistance, so constant-resistance pad would be preferable to just a variable resistor - if you want to keep the ballistics.
There are many cheap meters called "VU" meters that don't meet the spec but can still be useful and fun.
