Query if app is currently displaying Direct-to-HMD

Really quick question: I'm wondering if there's a simple way to query whether or not anything is currently being displayed on the rift via Direct-to-HMD? To clarify, I'm not asking "Am I in Direct-to-HMD mode" (that's easy: hmd->HmdCaps & ovrHmdCap_ExtendDesktop) but "Has ovrHmd_AttachToWindow been successfully invoked."