How to see real destination IP using tcpdump?

Question

I'm a newbie with tcpdump. My setup is simple:

• 1 machine A behind a router B
• 1 machine C outside, on the Internet

On machine A, I ping machine C, and I want to track what's happening with tcpdump:

• tcpdump on machine A shows an ICMP packet from IP (A) to IP (C)
• Then tcpdump on machine C shows an ICMP packet from IP (B) to IP (C)
• Then tcpdump on machine C sends an echo reply from IP (C) to IP (B)
• Then tcpdump on machine A shows an echo reply from IP (C) to IP (A)

So, at some point in router B (I cannot run tcpdump on the router), the IPs are converted from IP (B) and IP (A). I assume that the "real destination IP" = IP (A) should be stored somewhere deep within the ICMP packet, so that the router can redirect it correctly. But I can't find any option of tcpdump to show this "real destination IP" when it's run on machine C? What is this option, or how can I see it?