Tool for traffic measure?

[Sdävtaker]

Hello,
I got a subnet with 5 machines and a cablemodem who provides 5 public ips
All is conected to a switch.
One of the machines is not ours and we want to check it is not
abuseing our internet link, so we want to know if there is any way to
monitor bandwich usage from one of the other machines in the subnet
with no need to modify the foreing machine config. Something like use
tcpdump in promiscuos mode or something like that, we doesnt matter
the content, we just need a bandwich conssumption meassure.
Thanks for any ideas.
Sdav