The easiest way to detect current would be to get an ammeter in line with the circuit, which would mean using the actual wires instead of the USB connector. Personally, I'd get rid of the USB connector and deal with the wires.
Sounds like a cool project - gives hands-on feedback to textbook...
Just be careful. I have been inundated with salvaged parts, and have lost all storage area in my shop. It's a beast that can quickly get out of hand - suddenly everything you look at "has potential".