Easiest method is to put your multimeter into mA mode, and put it in series with the Positive power lead into the board.
Doing it in software, put a shunt in series with the power lead, use the arduino to do analog readings on both sides, and do the math: (Vhigh - Vlow)/shunt resistance, with Vhigh = analogreading * 5/1024 (or ~.00488)