I've already admitted to being a bit weak at the analog side of the house.
There's a product called Kill-A-Watt that you can insert between an appliance and your wall, and it will tell you the total power in watts (wall volts * amps), or just the current in amps being drawn.
What would it take to similarly compute this, say, between my handy 9V adapter and the Arduino (which may have peripherals like LEDs wired)? Is there a way to do this without an ammeter feature on a cheap volt/ohm meter? If I do find an ammeter, is it really as simple as putting the meter in series with the power source, or am I overlooking something?
I'm interested in the total current draw of my Arduino-based device, so I can start evaluating alternative power sources like solar panels, supercaps, etc.
Yes, what you describe will work. (I've even done that with the power wires on a USB cable.)
Another way is to put a low value (say 1 ohm) resister in series and measure the voltage drop across it and then use I=E/R to calculate the current. However the meter should be more accurate.
As far as I know, it shouldn't matter if you splice into the positive or negative wire, however, I would use the negative if I could.
f I do find an ammeter, is it really as simple as putting the meter in series with the power source,
Yep, that's all there is to it. Most DVMs have a current mode that requires you to move the + lead to a different connector.
A couple of notes: ranges are frequently small. Start on the high range (e.g. 10A on my meter) to make sure it doesn't exceed the capacity of the low range (400mA on mine) before testing the low range. Both are fused to protect the meter.
Also, make sure you switch the leads back to the voltage jack before testing voltage. The ammeter circuit is extremely low resistance, which leads to an extremely high current for a very short period of time (i.e. until the fuse blows) if you forget to change the leads back and measure, say, a vehicle battery voltage. I can also tell you that the fuse for the mA range in a Fluke meter is an odd size, a bit difficult to find, and a little expensive.