One of the most important tools needed to be sure your running IPSEC
correctly is tcpdump, which lets you look at whats actually
happening on the wire. To get a tcpdump that knows about IPSEC
packets and how to print out network traffic in ASCII we suggest
getting a new copy of tcpdump from http://www.tcpdump.org.
For a nice GUI when looking at IP on the wire, we suggest using Ethereal.