Is this a directly connected eBGP peer? If so, your directly connected IP facing this peer more often than not. However, if this is a multihop session it could also be using a route toward your loopback IP.
If the remote eBGP peer is already configured, you should be able to use traceoptions and sniff out what the eBGP peer is configured to peer with IP wise.
set protocol bgp traceoptions flag packets detail
set protocol bgp traceoptions file bgp-find-ip