Exchange team released an important monitoring script called CheckDatabaseRedundancy.ps1 to the Exchange Team blog.  The script monitors the redundancy of replicated mailbox databases in a DAG by validating that there is at least two configured and healthy and current copies, and it generates alert events when only a single healthy copy of a replicated database exists (e.g., a ‘single copy alert’).

We recommend running this script regularly, as part of your normal monitoring operations.  This is especially critical in JBOD environments. It is very important that all customers monitor for situations in which database redundancy is compromised so that they can immediately take action to restore database redundancy and avoid catastrophic data loss.

More information can be found at http://msexchangeteam.com/archive/2010/05/20/454976.aspx