Typically, your user sends network throughput problems reports as they see problem with their applications such as:
[a] FTP Transfer
[b] NFS Performance
[c] HTTP / SMTP / POP3 etc
As a sys admin you should able confirm the throughput problem. iperf is the tool you are looking to find out:
a) Network throughput problem
b) Packet loss problem
c) Datagram loss
d) Delay jitter
From the man page:
iperf is a tool to measure maximum TCP bandwidth, allowing the tuning of various parameters and UDP characteristics.
iperf works on client / server model. You need to install iperf on both client computer and server computer to measure network performance between two nodes.