Even easier is the Kill-A-Watt, just plug something into it, and it'll tell you the device's current draw, and usage over time. They even make a power strip version.

Though if you want to know the draw of an AVR before you have it in your possession that's usually listed in the back of the manual (which you can find in PDF form from most maker's sites).

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris