If they haven't contacted AD, I'd say it's safe to assume that they probably haven't contacted SCCM either. You can use this WQL Query in sccm:
SMS_G_System_CH_ClientSummary.ResourceID = SMS_R_System.ResourceId
SMS_G_System_SYSTEM.ResourceId = SMS_R_System.ResourceId
SMS_G_System_CH_ClientSummary.ClientActiveStatus = 0
That'll find all inactive clients. So you can set your threshold for inactive clients to 45 days.
I did a bit of research and it looks like this should get clients that haven't logged into AD in 45 days. I haven't done extensive testing with this query, so do some spot checking to be sure it's getting what you need.
DATEDIFF(dd,SMS_R_System.LastLogonTimestamp,GetDate()) > 45