We had mentioned a perl script in a former post that can be used to check network flow rate in command line.
Since python is very popular today in system administration, we will show a python script on this. Here is the output from this script:
$ python netio.py Current In: 0.75k/s, Current Out: 0.16k/s Current In: 0.15k/s, Current Out: 13.53k/s Current In: 0.06k/s, Current Out: 0.12k/s
You can get the script here: http://www.admon.org/tools/netio.py. By default, this script check eth0 with ifconfig for details, you can change the interface as you need.