Is there any way that, by looking at a bit stream, for me to know if I'm using the right bitrate?
Logic analyser, scope, one of the serial sniffers around.
Failing that just start trying different options (although if you don't know what data to expect it will be hard to know when you get it right).
or whether the bits are inverted,
Is this RS232 or 5v logic signal?
Measure the voltage during the 32sec idle period to determine the polarity. That will remove one variable.
Rob