How do I tell a serial port settings, when the hardware documentation doesn't have any info about it?
The most direct way is to look at the signal on an oscilloscope, assuming the device is emitting a signal to begin with. (Bonus: This will also help determine the correct pinout of the connector.)

You can easily determine the bit period (1 / the baud rate), the word length and how many stop bits are being used.

Determining the parity setting requires a bit more work, and it might be easier to just try the different parity settings once you have the other parameters set correctly.