How to interpret AlwaysOn event hadr_db_partner_set_sync_state
According to How It Works: Always On–When Is My Secondary Failover Ready?, a commit_policy of

DoNothing 

is an indication that the secondary is offline.  A commit_policy of 

Delay 

indicates that the secondary is online, but catching up.  And finally, 

WaitForHarden 

indicates the secondary is caught up with the current transaction.