ntpstat

root@h1:~# ntpstat
synchronised to NTP server (196.10.52.57) at stratum 3
   time correct to within 62 ms
   polling server every 64 s

S1 runs Debian Wheezy, so I can't install ntpstat... :-(

S2 and S3 run Jessie, so S2 is fine, but S3 is not running at the moment.
root@s2:~# ntpstat
synchronised to NTP server (197.80.150.123) at stratum 4
   time correct to within 298 ms
   polling server every 64 s