In UNIX the netstat command helps you as an administrator look for potential bottlenecks. With the following flags netstat is a powerful tool in determining bottlenecks.
-i for a specific interface
-n displays network address as numbers
-r displays the routing table for the machine
-s lists per protocol stats for UDP, IP, TCP, and ICMP
Solaris also comes with programs like ethers that is a GUI that shows network connectivity
Windows NT uses Perf Mon (discussed in System Resource Monitoring) to watch NBT connections to the local machine from other hosts. It gives you Trans/sec, Receives/sec and Total/sec.
Although this isn't as powerful as netstat for UNIX it is a good way to keep an eye on the average bytes/sec in and out of your NT box. A program that will do the job is NetMon .