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:
select
SMS_R_SYSTEM.ResourceID,
SMS_R_SYSTEM.ResourceType,
SMS_R_SYSTEM.Name,
SMS_R_SYSTEM.SMSUniqueIdentifier,
SMS_R_SYSTEM.ResourceDomainORWorkgroup,
SMS_R_SYSTEM.Client
from
SMS_R_System
inner join
SMS_G_System_CH_ClientSummary
on
SMS_G_System_CH_ClientSummary.ResourceID = SMS_R_System.ResourceId
inner join
SMS_G_System_SYSTEM
on
SMS_G_System_SYSTEM.ResourceId = SMS_R_System.ResourceId
where
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.
select
SMS_R_System.Name,
SMS_R_System.LastLogonTimestamp
from
SMS_R_System
where
DATEDIFF(dd,SMS_R_System.LastLogonTimestamp,GetDate()) > 45