ip addr sh dev eth0 at the command prompt and look at the output to determine which eth0 interface has the Shared IP address (eth0 of the primary or eth0 of the secondary):
> ip addr sh dev eth0
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:30:48:79:62:24 brd ff:ff:ff:ff:ff:ff
inet 192.168.5.231/24 brd 192.168.5.255 scope global eth0
inet 192.168.5.230/24 scope global secondary eth0
In this example the shared IP address is 192.168.5.230. The eth0 on the secondary has the shared IP address.
Using a shared IP address in your high availability configuration is optional.