To get a number that means anything it's going to take a lot more time, equipment and money than you want to throw at it for a gee whiz answer.
Manufacturers use a huge power supply with selectable voltage and a current readout. More importantly they use a precision load that is not reactive (no capacitance or inductance value) and is precisely adjustable. The non-reactive part is very important (and also makes real world application info impossible to determine from the manufacturer's data). Because the load is non reactive the load seen by the amp does not vary with frequency or power applied (nothing like your subs). Because is doesn't vary, it also doesn't induce a phase shift between voltage and current. The power number that they get actually is V*I applied at the same time.
Now step back to the real world. You can wire your subs to a nominal load but the amp is never going to see that load. You also get a phase shift between voltage and amperage such that the peaks don't align. If you were to look at definitional power applied there would be four points in each cycle where the power applied would be zero (points where either voltage or current was zero). That said, it's the current that moves the driver. The voltage induces the current. Even though there is a phase shift between the two, they are still correlated by Ohm's law. You can get an RMS value for voltage and for current and multiply them and get what would be considered power though because of the phase shift the correct term would be volt*amps.
If you are expecting to get rated power numbers for the nominal load that you have your subs wired to if you test this, be prepared to be greatly disappointed. You'll be getting much less because of box rise and power compression.
