ASA 5540's configured for active/passive failover.

Primary loses connectivity, failover occurs, Secondary is now active.

To the untrained eye, how can you tell which device is the Primary?

i.e.

If the outage was from something other than a hardware or physical link failure, say the VLAN changed on the port that the Primary is on, but the connection is still up... link lights would show, power would show, etc.

How can you tell which is which?

Thanks

(a bit new to the failover fray)