Via the internet
There are a few utilities that can help. Object Development’s US$30 Little Snitch (www.obdev.at) includes a network-monitoring function (find it in Little Snitch Configuration’s preferences). In the window that appears, you’ll see a list of all the applications that are currently accessing the internet, with meters next to each one that light up when data is passed back and forth. The application that’s biting off the greatest portion of your bandwidth appears at the top of the list (see ‘Snitching on bandwidth hogs’). If it’s an application you can do without, or configure so that it’s not hitting the internet so often (or with such force), make the necessary adjustments.
ProteMac’s $28.32 Meter (www.protemac.com) is a network-traffic logger. With it you can generate charts of traffic activity (thus looking for spikes and lulls), and monitor application traffic in real time.
Finally, there’s Conceited Software’s Rubbernet ($51.99, Mac App Store). It, too, monitors your Mac’s network. What it gives you in addition is the ability to monitor other computers on your network. Just install the Rubbernet daemon on those computers, and you can see what they’re up to. You can then use this information to diagnose local network-wide slowdowns.