Where is the MAC address of my Thunderbolt to Ethernet adapter? I have a Thunderbolt to Ethernet adapter for a MacBook Air and I need to assign a static IP address to it.

I assume the MAC address is embedded in a given adapter... and by convention I would assume that the MAC address would be written somewhere on the device, but it is not.  Does the MAC address have more to do with the adapter of the MAC itself?

In other words if I swap adapters will the MAC address change, or will it stay the same?  What if I swap thunderbolt ports it's plugged into (for MacBook Pro Retina)?

Does the same logic above apply to Thunderbolt displays since they have Ethernet?
The MAC Address is unique and a property of the actual network hardware (although it can be spoofed). So your adapter has its own MAC address, as does the the network interface in a Thunderbolt display, and the Wi-Fi in your MacBook Air.

Different Thunderbolt to Ethernet adapters will have different MAC addresses, but moving one between different ports won't change it.

You can view the MAC address of any network interface attached to your Mac by using Network Utility, which you'll find in the Utilities folder within your Applications folder. It calls it a Hardware Address, but it's the same thing.