Correct, if you open SE16, display table RFCDES you see a list of defined RFC destinations.
The field RFCOPTIONS gives a list of the attributes. You will see something like:
H=....,S=..,M=...,U=xxxxx
These contain the attributes of the RFC destinations
H: Host
S: Instance ID
M: Client
U: the user ID used in the RFC destination. So here you would find the required User ID.