How can I test the amount of power I can draw out of a USB charger?
The simplest way is to give the charger ever increasing loads until it no longer supplies close to 5 V.  That won't tell you what it can always do under all combinations of line voltage and temperature, but it should give you some idea.

The easiest way to load the charger is with known resistors.  A 10 &Omega; resistor will draw 500 mA when 5 V is applied to it.  Note that it will also dissipate 2.5 W in the process, so this would need to be a power resistor.

Since 2 W is a common power resitor value, you could get a bunch of 15 &Omega; 2 W power resistors.  Each connected to 5 V will draw 1/3 A.  Keep connecting more of them while monitoring the voltage with a voltmeter.  Count the number you can put accross the supply before it either shuts down, blows a fuse, or otherwise can't hold the output at 5 V anymore.  At 1/3 A for each resistor, you get a good enough idea.  You probably want to derate the limit you measured by 1/3 A (one 15 &Omega; resistor worth of current), maybe more if the charger blows a fuse instead of current limiting or shutting down.