Amp Output Testing
The easiest way to test is with a pal; one guy to operate the source unit and the other to verify that both readings were taken and que when to start and stop the test. A great way to test is by warming up the subs and amp to get readings congruent with how you actually play the system.
Once they have played some music at moderate volume for a while, your measurements will be more accurate and define what is actually happening when you are enjoying the system. With both meters in place and set correctly, burp the system at the head unit volume level that is just under clipping indication on the amplifier (as relayed to you by your friend). The Voltage and Amperage numbers are simply multiplied together to get your RMS Wattage. (P=VI or Power equals Voltage times Amperage). To find out what impedance your amplifier is seeing, simply divide the Voltage by the Amperage. (R=V/I or Resistance = Voltage divided by Amperage). If you want to plot a power or impedance curve, simply repeat the previous test for each frequency, i.e. 70, 65, 60, 55, 50Hz etc.
Most standard or manufacturer recommended enclosures will lend a pretty substantial rise to the average impedance of your subwoofers, which means that the amplifier will push significantly less power than what you might expect.
An informative experiment would be to test your box as it sits and then push it against the back of the boot or cargo area; maybe 1inch from the port opening and test it again. Your numbers will come out way different.