Check NFS mounting on the server and the NFS version

Check version

# check the versions currently supported on the NFS server
cat /proc/fs/nfsd/versions
# output: -2 +3 +4 +4.1 +4.2

Check mounting status

# If there are clients mounting NFS with versions < 4:
rpcinfo -p | grep "tcp.*mountd"
# output: shows the PID, protocol and port number. E.g.:
# 10005 1 tcp 33981 mountd
# 10005 2 tcp 51769 mountd
# 10005 3 tcp 48233 mountd

For NFS v4, only port 2049 (both TCP and UDP) shall be used.