Sounds like the typical Arduino-ish setup where you have a USB-to-serial chip talking to some microcontroller's UART. The diagnostic tool of choice would be a serial terminal (I recently discovered the absence of HyperTerm in Windows 10, but remembered PuTTY could speak serial), preferably in combination with some sort of documentation for the serial protocol so you know what to send in the hope to get it to respond. Sadly, the nature of asynchronous serial comms is that it can be a right pain to work out the connection parameters when you're dealing with something that only transmits in response to valid data. If you're lucky it'll spontaneously spew data at intervals without you talking to it, and if you're really lucky it will be human readable (or at least intelligible ASCII).
FWIW, I'd be astounded if it wanted anything but 8 data bits, no parity and one stop bit. No flow control is likely.
If it's just a question of which COM port is associated with the USB interface, I thought Device Manager showed such things...
