iPerf is a tool for active measurements of the maximum achievable bandwidth on IP networks. It supports tuning of various parameters related to timing, buffers and protocols (TCP, UDP, SCTP with IPv4 and IPv6). For each test it reports the bandwidth, loss, and other parameters. It exists for multiple platforms, but not officially for VMware ESXi. Apparently a binary compiled for RHEL5 can still be run on any ESXi host from 4.1 to 5.5. Download and extract the file to your ESXi host and execute it. If it is not executable set the permissions accordingly.