How to find out whether this STM32 board can communicate through its USB port by default?
The product page for Nucleo-F302R8 says the following:

On-board ST-LINK debugger/programmer with USB re-enumeration capability: mass storage, Virtual COM port and debug port

It means the USB port is for the on-board ST-Link programmer, which shows by default as three devices, the JTAG/SWD programmer, the USB drive to program it, and as a debug serial UART port.
So yes, it can be programmed/debugged and communicated over serial connection simultaneously. You might want to read the manual for further details how the board works.