sudo ipfw -a list
the -a means include the byte counters in the list as well. If you're a slacker like me, you won't have your firewall turned on (I do have a router/firewall between me and the big bad internet though), and all you'll see is the one default rule:
65535 1159997 393855140 allow ip from any to any
The numbers mean:
rule number 65535, which has matched 1159997 packets, for a total of 39385140 bytes, is "allow ip from any to any"
Note that, if you have a fancy rule set that uses dynamic rules, you'll be missing much of the traffic, because the rules that matched it will have disappeared from the current list