It is possible to test whether a leap second will be used. Due to the nature of NTP, the test might work up to 24 hours before the leap second. Some major reference clock sources only announce leap seconds one hour ahead of the event. Query the NTP daemon:

% ntpq -c 'rv 0 leap'

Output that includes leap_add_sec indicates proper support of the leap second. Before the 24 hours leading up to the leap second, or after the leap second has passed, leap_none will be shown.