If you need to determine if a circuit or power supply can deliver on its promised current capacity, then a quick way to test this parameter is with an electronic load. You can of course purchase your own commercial version, or make your own - and one example of this has been demonstrated by Jakub Polonský and his MightyWatt. In his own words,
... the basic idea is pretty simple. I connect the device under test to a FET and shunt resistor to measure current. The gate of the FET is driven by an op-amp that maintains either constant voltage across the load or constant current through it. It all attaches to Arduino Uno R3 as a shield.
The design is quite clever, and can offer a constant voltage or current, and oversamples the Arduino's ADC to 12 bits for voltage and current measurement. Although the MightyWatt leans towards the more complex end of the project spectrum, Jakub has published the design files so you can have your own board and thus make your own version.
If you're interested in experimenting with Arduino for your own inventions - but not sure how to learn - order one of our Experimenter's Kit for Arduino:
The package includes a wide variety of parts, sensors and modules including: a servo motor, lights, buttons, switches, sound, sensors, breadboard, wires and more. Furthermore a Freetronics Eleven Arduino-compatible board is included to make this an extensive hobby experimenter, inventor and starter kit.