How to determine spi pins?

[sirtet]

So, I have this Bluetooth module with a piggyback pcb on it, and on that pcb is an soc (csr8645), that has an spi port, but I can't tell if the spi pins are on the pcb, cause it's a bga chip.
There are 4 pins on the piggyback pcb (pins 12-15) that i guess are the spi ones, because
they go to test points on the main pcb bottom side.
But how to tell which is which?

Is there a  better approach to figure them out than trying lots of combinations,
Sending a command and probing for an answer?

[bitwelder]

step 1. Plug a logic analyzer (or an oscilloscope that can also decode SPI) and observe?
step 2. Make guesses on the pins function and try to decode valid SPI messages?