Hi Ashish,
I don't think this would be the correct approach. SM59 would have RFC destinations which would be used by SAP system to connect to the other systems, not the other way round. When an external system makes a call to my SAP system it would use a destination created at there end.
And identifying RFC's in the system would not be of much help, as there would be a lot of RFC FM's, but which are used in the SAP system only, like BAPI's.
Regards,
Anirudh