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.
Managing OS X 
It goes up to 4 I believe, but it’s almost useless when you set it to be that verbose. 🙂
defaults write com.apple.MirrorAgent printTrees 1
will also print the internal state trees which is quite illuminating.
THANK YOU THANK YOU THANK YOU! I had a bunch of clients stop syncing and I’m going nuts trying to figure out why
To be honest, I haven’t found these debug settings to be that useful in the times when I’ve found clients to stop syncing altogether, but they have been really useful when trying to work out why some things are syncing incorrectly… 🙂
I found most of the people that have stopped syncing have had to be removed and readded to the kadmin.local. The onesi have removed and readded start syncing again.