With RS232 you can hook up an old oscilloscope and measure what the right baud rate should be. Even in the "won't transmit unless it receives" case, a sweep on a waveform generator and well configured trigger will get you on your way. USB you'd need much nicer tools handy to get to the bottom of an issue.

You are right on about the joy of it being 9600 8N1 on the first try.