Brian,
You can usually find this information in the CRM database. An activated record will have a StateCode value of 0, and a deactivated record record will have a StateCode of 1. Further, you can usually tell the reason for the deactivation on certain types of records based on the StatusCode. StatusCode is unique per type of record. I'm not 100% sure how it works for Custom entities.
But I know for example with Opportunities you can tell the reason an Opportunity is closed based on its StateCode and StatusCode (closed as "Won", or closed as "lost", reason "Sold out", etc"
http://technet.microsoft.com/en-us/library/ms914670.aspxSee the above link for out of box entities' State and StatusCodes and if you are familiar with querying the database maybe you can infer the reasons for the deactivations based on your custom entities' status codes. Good luck
Deactivation Status Reason OnSave
