If you are measuring a USB cable expecting RS232 TTL serial communication, then you're wasting your time. If you really, really want to see 5v on a USB cable, try the red and black wires. Try not to damage your scope.
USB is serial, but it's not RS232 serial. You're intercepting signalling between the host and the any number of devices including all the USB protocol negotiation on differential signal lines. You can have more than one device on a USB root hub - and all this has to work in harmony somehow.
What you're referring to - being the "serial" that is mostly used by MCU's doesn't travel along the USB cable bare. Often the "serial" portion is on a chip on the Arduino board, or is baked into the MCU. Other times, people use an FTDI adapter.
Trying to work out the baud rate of encapsulated (if you could call it that) serial communications over a USB cable is meaningless. Your USB device may be operating at USB 3 speeds, but only communicating serial data at 9600kbps. For all you know, your USB 3 serial device might be operating at 5gbps of "not doing all that much".
This is like jamming a cable internet coaxial lead into an laptops RJ45 port and wondering why there's no wifi.
You might want to check out USB Made Simple - Part 1 for some more specific details on USB communication.