There is no set list of possible baud rates when opening a serial port. Instead you can check the return code when using serXopen (with X standing for the particular serial port).
For example, a return of 1 when using
rc = serDopen(115200);
means that you were able to open the serial port at 115200 baud.
A return of 0 means that it was not successfully opened at that baud rate. I believe this is within/without a 5% tolerance.