Hi,

Is it possible in SCCM or in SQL to query which clients have offline files enabled on their machines? TIA

Not by default no. But you can query the appropriate registry value using hardware inventory or using compliance settings.

To add a registry value to query, see http://be.enhansoft.com/post/2014/02/13/How-to-Use-RegKeyToMof.aspx

To use a compliance setting, see http://blogs.technet.com/b/smartinez/archive/2013/07/31/creating-a-compliance-item-baseline-and-example.aspx

In 2012, it's generally preferred to use compliance settings for something like this as it is a bit easier to target and create within the console but both are effective and work well.