NTP servers are normally set in /etc/ntp.conf.
It may take up to 4 hours of semi-continuous reachability to calibrate the clock before you achieve right stratum status. If the stratum status hasn’t changed in a few hours, your synchronization is definitely failing.
From synchronization with stratum 2 servers, your stratum should settle at stratum 3.
You can check your stratum status (and other info):
ntpq -c readvar
You can check what peers you are connected to (and in turn what they are connected to):
ntpq -c peers
Check whether you’re synced (should see stratum=3):
ntpq -c rv | grep stratum