I stumbled across this undocumented tidbit on the Apple Discussion forums:
You can get debug info written to ~/Library/Logs/MirrorAgent.log by doing this:
defaults write com.apple.MirrorAgent debugOutput X
Replace X with 0 to turn off debugging. I’ve tried 1 and 2, and 2 gives more detail than 1, but I have no idea how high you can go.
With debugOutput set to 1, a logout sync generated over 700 lines of output to ~/Library/Logs/MirrorAgent.log.