What are you trying to drive with those pins? I=V/R, if you know the resistance of whatever you're connecting to those pins you can calculate the current pretty easily.
Yes, you can measure the current with a multimeter. Just make sure to connect it in series with whatever load you have on those pins not in parallel, otherwise you'll just short the GPIO output.
Edit: Did you mean the max current the GPIO is capable of providing? If so, then as said below, a google search tells me it's 16 mA per pin, with a max of 50 mA total for all pins.