Testing FT230X USB i/f - best way?
Do a hardware loopback.  If the following doesn't work, then either your device has problems or your driver has problems.

1) Connect TX and RX pins together.

2) Start terminal emulator software.

3) As you type characters, they will be transmitted then received back into your computer, then displayed on your screen.  If you have ECHO enabled in your terminal emulator then you will see 2 of every character you type.

4) To test a large amount of text, "upload" an ASCII text file, but first set the terminal software to capture everything.  Clean up the captured file, then compare against the original file that you uploaded.