Identifying network bottlenecks is generally fairly straightforward any network link will have a finite amount of bandwidth and the higher the proportion of this that is used up the more applications will be slowed up in communicating with other devices on the network. The actual value of utilization that will indicate a serious problem will depend on the network topology with switched networks the utilization level can be significantly higher than can be sustained in shared networks, so whereas anything over about 30-40% in a shared network will indicate a problem, that wouldnt be the case in a switched environment.
Modern PCs and servers have sufficient performance that they can saturate a network link quite quickly if they are performing sustained network activity such as large file copies. For each individual PC the impact is limited to that PC but of course a server has to service requests from all the other machines on a network, so it is a point of concentration and if there are several client machines making heavy network I/O demands on the server these must all be channeled through the server’s NIC and the switch port into which it is connected.
There are a number of ways to overcome network utilization issues including installing higher throughput components, such as Gigabit Ethernet devices, network reconfiguration, to use multiple network segments to divide up the traffic across multiple network interfaces, and teaming of network interfaces, to utilize more than one physical network port on a single subnet. Which solution is practical and appropriate will require a detailed understanding of the current network topology and application environment.