There are several methods for monitoring bandwidth usage, with different levels of granularity.
If a connection is currently active, you can connect to your pfSense router's console (physical access or ssh) and watch the traffic flow with pftop (Option 9).
Install iftop as demonstrated on [Installing FreeBSD Packages]
Run it from the shell using something like:
iftop -nNpPi em0
Press 't' to cycle through various views.
Another option for viewing real time throughput is trafshow. To install it, log in via SSH, choose option 8 and run:
pkg_add -r trafshow rehash
To run it, at a SSH command prompt, run:
Then select the interface.
If you are only interested in overall per-interface usage, there are built-in RRD graphs on pfSense, which can be found under Status > RRD Graphs.
If you need a little more detail, such as by client IP on your LAN interface, there is a package for bandwidthd that you can install under System > Packages. Once installed, it appears under Services > BandwidthD.
Darkstat is also available in System > Packages. Once installed, it appears under Diagnostics > darkstat. It also offers bandwidth graphs for an interface, as well as traffic to/from specific IP addresses.
If you need even more detail than that, you might need the ntop package, which can also be found under System > Packages. It can break down detail by IP, protocol, and so on. Once installed, it appears under Diagnostics > ntop. It will even track where connections were made by local PCs, and how much bandwidth was used on individual connections.
Currently, darkstat and bandwidthd do not listen on multiple interfaces. ntop will listen on multiple interfaces, but the WebGUI may not show both selected in the ntop settings page. You can confirm it is listening on multiple interfaces by going to Diagnostics > Command and typing:
ps xa | grep ntop
You should see a line that looks like:
$ ps xa | grep ntop 59649 ?? Ss 0:06.17 /usr/local/bin/ntop -i fxp0,ng0 -u root -d --ipv4 -M
Note that if you make changes to the settings, you should probably reselect the desired interfaces before pressing the Save button.
Netflow is another option for bandwidth usage analysis. Netflow is a standard means of traffic accounting supported by many routers and firewalls. You need a Netflow collector running on a host inside your network to collect the data. pfSense can export Netflow data to the collector using the pfflowd package, or softflowd.
vnstat is another bandwidth monitoring tool available to install as a package. See the Vnstat doc for more information