You need an amprobe multimeter.http://www.amazon.com/Amprobe-ACDC-100-Digital-Clamp-Multimeter/dp/B001C6P38U
Basically, you just pass the yellow wire through that clasp, and it will tell you how much amperage is being used. Multiply that amperage result by 12v to get wattage.
But the card will also draw some electricity through the PCIe slot, and you can't measure that with an amprobe.
Another possibility would be to cut the outer insulation from the three-wire PSU cable, then pass the amprobe through one of those wires. That'll give you power consumption for the whole rig.