Check network interfaces


I have installed nagios and configured a lot of what I want to monitor. There is one thing I can’t figure out how to do though. Here is my scenario:
I have two virtual servers. Server A and Server B.

They are configured in a manual failover. They each have their own unique IP address. They also have an IP address that they share. Both servers have a NIC assigned to them with the same MAC address and the same IP address. We simply disable and enable the NIC to failover the services. I want to monitor which server the failover MAC/NIC/IP is enabled on. On both servers eth0 is their own interface. Eth2 is the identical/shared interface. Can you guys give me some tips on how to do this? I am new so can you be as descriptive as possible? Thanks!