how to know if the traffic via IPSEC tunnel is encrypted and tunnel working

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http://www.linuxquestions.org – Hi all, i created ipsec tunnel between my hardware firewall and linux machine with openswan, its working fine, now i just want to know that the traffic via created tunnel is being encrypted and tunnel actually working. please let me know how to do it is there any tool to check, i heared of tcp dump but i don't know how to use if tof this task. thanks in advance. (HowTos)