I2s out Xilinx xc2c64a vq44

I have an unlabeled usb to i2s converter using this chip. Anyone know how to identify the correct output pins? I think pin 15 may be data out and 12-15 are the pins in question, but that is as far as I have got and I am not sure it is correct. Any insight would be appreciated before I embark on an annoying trial and error process.
The chip is a CPLD; depending on its configuration file, any of the normal digital I/Os can be anything.

According to its datasheet, https://www.xilinx.com/support/documentation/data_sheets/ds311.pdf , pins 12...14 and 16 are normal digital I/Os, but 15 is not (see pages 10 and 11, VQ44 column). On page 12, pin 15 is identified as an internal power pin.

Do you have an oscilloscope? The bit clock and word clock should be easy to identify with a scope; the word clock should be a square wave at the sample rate and the bit clock a square wave at a much higher frequency, typically 64 times the sample rate - assuming the I2S output operates as a master, which is quite likely.