How do I know if data from compass are used?

Hi, we did port embedded motion app platform code for MPU6050 to JN5148 with a good success. Device is working pretty good (still under heavy test right now). We connected an HMC5883L compass device to the MPU and we rewrite ak8975.c interface to adapt to HMC5883L.
We did all is stated in documentation in order to use compass device, but how can I be sure compass device data are used?
To read data out of compass device, is the API function "ext_slave_descr.read" (which is a function pointer to our hmc5883l specific implementation) used? So I can put a log therein?

Thanks for any response.
Paolo.