Hi Nikolaus,
in the structure mapping part of /aif/cust you can assign the fixed values and data checks. There you can also assign an action which is basically a function module that has to adhere to a certain signature. You can use this action to call your rfc and maybe handle some kind of result by marking the call as failed in /aif/err.
This way you will have monitored each call to the rfc if it goes through your interface including the data passed and means to manage the call like restart it or change data and so on.
There is no dedicated RFC Monitoring step by step manual inside the cookbook but if you fully read it you should be able to create a solution that fits your needs in the way i just outlined.
Kind regards.
David Rittinghaus