The backbone of the highavailability is the nodes in a cluster checking each other so you should look into heartbeats. They are usually implemented by sharing a disk space, like a concurrent accessable VG, which is rather small and have the nodes write in there some bits and by the freshness of it the nodes can decide who is still up and alive.
Additionally there is heartbeating via network interfaces. Some even use or used serial interfaces etc.
This is an important part of HACMP/PowerHA and other Cluster Technologies.
Have a look here:
Heartbeating in HACMP - AIX 6.1 Information Center