How can I detect which baudrate a virtual COM port is set to?
From looking at the source it appears that on 32u4 based boards Serial  includes extra methods to access the settings from the USB host:

see: https://github.com/arduino/ArduinoCore-avr/blob/b7c607663fecc232e598f2c0acf419ceb0b7078c/cores/arduino/USBAPI.h#L129

From USBAPI.h:

// These return the settings specified by the USB host for the
// serial port. These aren't really used, but are offered here
// in case a sketch wants to act on these settings.
uint32_t baud();
uint8_t stopbits();
uint8_t paritytype();
uint8_t numbits();
bool dtr();
bool rts();