How to determine if NFS mount is mounted as v3 or v4?

Here are 2 ways to do it:

mount

Using mount's -v switch:

$ mount -v | grep /home/sam
mulder:/export/raid1/home/sam on /home/sam type nfs (rw,intr,tcp,nfsvers=3,rsize=16384,wsize=16384,addr=192.168.1.1)

nfsstat

Using nfsstat -m:

$ nfsstat -m | grep -A 1 /home/sam
/home/sam from mulder:/export/raid1/home/sam
 Flags: rw,vers=3,rsize=16384,wsize=16384,hard,intr,proto=tcp,timeo=600,retrans=2,sec=sys,addr=mulder

Use nfsstat -m it will display all the nfs mounted filesystem and theirs properties.

Don't forget the ultimate authority!

cat /proc/mounts