You can traceroute to something on the far side of the router and see what it reports when the path goes through it.
If you can get into an adjacent Cisco device you can usually see the IP addresses of other adjacent Cisco devices with the command "show cdp neighbor detail". Among the things that will report (assuming CDP is not disabled) is the management address of the neighbor router.
You can also connect to the router's physical console port and check the config directly.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...