The easiest way to tell whether an STU-540 is operating serially or as a standard USB device is to look in Device Manager. If it is configured to operate as a standard USB device then it will appear in the list of USB devices as shown in the first screenshot below.
If it is currently in serial mode then it will appear as a USB Serial Port under "Ports (COM & LPT)" - see second screenshot.
The COM port number will vary depending on how many other serial devices are already connected to the PC.
If there are multiple serial ports on the system then the getSerialPorts and Query tools can be used to help identify which one relates to the 540.