To use network stats command, you have to open the console first using the @ key, or sometimes the ` key (on my keyboard layout anyway).
The bottleneck field (top left), will usually show you the person who's bottle-necking (causing the lag), but this isn't always conclusive, as sometimes it flickers. In the table section of the netstats screen, the data behind column is also a good indicator of who's lagging.
To exit the netstats screen, just type ren_shownetworkstats into the console again and press enter.
More info here:
http://supcom.wikia.com/wiki/Console_commands