Note: This note was originally in the Network Booting a Raspberry Pi 3 from an Ubuntu Server post.
Initially when trying to understand whether the Pi was booting from the network or not it was useful to enable NFS debugging with
rpcdebug -m nfsd -s proc which reports quite verbosely to syslog.
Do disable run
rpcdebug -m nfsd -c
More details here.
Watching the file-system can also be helpful.