Network troubleshooting, Part I: What physical NIC does the VM use?

If you encounter a network issue in a VM (like bad performance or packet drops) a good first question to ask yourself is: Is this issue limited to the VM or can it be pinned to one of the host's physical NICs?
So, you need to find out what physical NIC (pNIC) the VM is actually using. In most environment this is not obvious, because the virtual switch that the VM connects to typically has multiple physical up-links (for redundancy) that are all active (to maximize bandwidth).

Unfortunately, it is not possible to find this out by using the VI client. It does not reveal this information regardless whether you use standard or distributed virtual switches.
You need to log in to the host that runs the VM (see the HowTos section for instructions) and run esxtop.
Press n to switch to the network view, and you will see a picture like this one:

Network view of esxtop (click to enlarge)

Find the VM's display name in the USED-BY column and look to the corresponding TEAM-PNIC column then. In this example the VM FRASINT215 uses vmnic1.