Can't find consistency? I am surprised. The ATP PTS is designed for consistency. All DPEs and inspectors should reference the document for their administration of the oral.
Basically, it's a systems oral. Unless your checkride is being provided by a 135 or 121 operator (since it's in a Duchess, sounds like that's not the case), that's about all you should expect, except maybe for some pt. 91 IFR questions. Know the systems of your checkride aircraft inside and out. Check the FAA ATP PTS at http://av-info.faa.gov/data/practicalteststandard/faa-s-8081-5d.pdf .