Also, any network-activity-monitor GUI Aqua utility can help you on that. I personally use 'Net Monitor', it is highly customizable.
Those utils tell you of the general in/out network activity. For example, if someone is uploading to your machine you will see a spike of inbound traffic. Net Monitor has the capability of monitoring several interfaces: ethernet, PPP...
It's free as well.
dani++
(PS: if you dig, you'll discover that the UNIX part of MacOSX eases the development of many such utils, which are *very* useful)