Here's what I do:
1. Install a bandwidth monitor for your connection. I use DU Meter.
2. Keep the bandwidth monitor in the corner of the screen and watch
your performance until you get a feel for what seems normal for
various sites and time of day.
3 To find bottlenecks, I'll use the same app to download from
different sites. i.e. If I'm seeing slow downloads from newsgroups,
I'll run multiple copies of Agent to different news servers, and see
if they are all slow, or just a specific one.
Before long you'll get a feel for how your connection behaves.