Identify NTP clients using my server

tcpdump port 123 ?

ntpdc -c monlist might also work as well, though I don't have a ntp server handy to test.

#ntpdc -c monlist

will show exactly what clients are being served by the NTP server. However, I suggest to do ntp service restart prior to listing. This is because, the list will contain old clients as well, including ones which might be listened long back and then removed from the network/decommissioned.

#service ntpd restart 
#ntpdc -c monlist 

This will give you exactly the current data.