Passive listening to USB communication
I'm not certain about this, but I believe that if you look at the differential voltage between the two lines, you will see data, but it will be NRZI encoded. This means that the bits are encoded by whether the bus level changes or not. Change (in either direction) means 0 and no change (for one bit-time) means 1.

I believe there are also CRCs and other signaling bytes at the beginning and end of packets.