Is there a special WAV file that could be recorded, where separate tones for each frame of 23.976 were repeated each second so that it would be easy to look at the first frame and the last and see if there was any drift?
Typically, I recommend that my clients shoot a camera test prior to the first production day where they try out the work flow: run the camera normally, clap a regular timecode slate at the head, roll for at least 2-3 minutes, then clap at the tail. Load the R3D files and the separately-recorded sound files into a laptop, convert the R3D to whatever the client plans to use for editing. Sync up the material and verify timecode matches and sound does not drift. If it's even 1 frame off at the tail, you'll know it.