dkl65:
So there isn't a way to measure the full current capability of a power source.
Not /strictly/ true. You can dummy load any power source you want and watch the power draw in-circuit
Ignoring the impedance of the meter (which may or may not be a good idea, depending on your meter!) and things like temperature, this implies something like a 5.5-10 Ohm resistive load would not trip the computer's 500-900mA (depending on hardware) over-current protection and allow you to measure it.
(This is just back-of-the-envelope on my part -- I'm actually hoping someone will correct me if I'm wrong.)
Again, though, probably not really helpful or useful in this context.
If you are trying to measure how much current the whole Arduino is drawing, you can do that with a USB breakout circuit. Otherwise, you can measure various points in your circuit to see what the draw is. Using Kirchhoff's law you should be able to work out where this is the same and where it might differ across a circuit.
This is something I've been trying to take into consideration more when I think about stuff where current is important -- designing in test points to make measurements easier.