Hello Luigi,
There really is no standard way. The Beckhoff module has got LEDs, so if you think that the IOs are configured, then just try out the first 30 or so and see if any LED goes on. Of course make sure that there is nothing else connected to the robot that could cause shorts or activate external machinery.
The proper way would be to start up WorkVisual, load the current project off the controller and then check how the physical and virtual IOs are mapped, or to map them yourself.
I've attached a screenshot below, in that case you can see that toggling $OUT[1] from KRL or KUKA|prc will toggle the physical output 9 on the IO module.
Best,
Johannes