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)|