The "real" wattage rating can be found on the ID plate on the back of your receiver. It will either be an actual wattage or Volt/Amp rating. Volts x Amps = Watts. Subtract 20 to 30 watts for system overhead and what is left is what you would use. Your manual might also have the Dynamic Wattage specs, which is usually stated with one or two channels driven and is useful when using just two channels.
If you want to get a rough idea of how many watts your system is drawing during a particular gain setting, then you will need an SPL meter, the distance from speakers, the sensitivity of your speakers, and test tones. There are formulas published on WikiPedia that you can use to figure out the wattage draw for one or multiple speakers driven.
Denon AVR X7200WA, Emotiva XPR-5, Emotiva XPA-7, Sony XBR 75" 850C, Oppo BDP-103D, Roku 4.
DefTech BP7000SC (L/R), CLR3000 (C), SM55 (FH), UIW BPZ/A (SB), UIW 75 (TM).
JBL L7 (Sur), EoSone RSR 350 (RH).
PC 2.1 setup. Emotiva Stealth 6, SVS SB2000 sub, Emotiva DC-1 DAC.