A leap second is an ad-hoc one-second correction to synchronize atomic timescales with Earth rotation.
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'
ntpdate -v -b pool.ntp.org
probably you will need to stop ntpd /etc/rc.d/ntpd stop
Use ntpq to see more info: