How to select a power supply for a given Android device
I'm not aware of any way to find out the maximum current the device will use except checking the charger that came with the device and assuming that the manufacturer suppied the most efficient charger suitable. 

The maximum current for USB connection is 5A, but usually the range used by phone/tablet chargers is between 0.5A and 2A. 

Note that the amperage of the charger only tells what is the maximum current that the charger can supply, so it's safe to plug a 2A charger to a device capable of 0.5A current.