Hello Manish,
In transaction SM58, you should be able to see RFC destination that is used by the caller system to access called system - for each erroneous or executing tRFC entry, this will be seen in the column 'Target System'. The identified RFC destination should be then checked in transaction SM59 (or you reach it by double clicking on the target system name directly in SM58). There, please check maintained user on the tab 'Logon & Security'. On this tab, you will either be able to see the specific user that is maintained in the destination and that is used by caller to authenticate itself in the called system, or, if the checkbox 'Current user' is active, then the RFC call coming via this RFC destination to the remote system, will be performed in the context of the same user of as the one invoking RFC call from caller system - the one that can be ideitified from the column 'Caller' in SM58.
Regards,
Vadim