Ntpd monitoring

NTP monitoring plugins exist for Nagios - see http://exchange.nagios.org/directory/Plugins/Network-Protocols/NTP-and-Time. The one I am using does not need SNMP, it just asks NTP server via NTP and compares information.

What should be the output of the monitoring tool? Which OS? Do you need to plug it in some NMS?

You can use the NTP plugin for Munin if you want to monitor NTP data over time; it will show you delay, offset and jitter. I've attached an example of the output.

Munin NTP output http://www.freeimagehosting.net/uploads/c10ed0a462.png

If you just want to see if your ntpd is running, what its synchronized to, etc, and you're comfortable with command lines, you can use ntpq. I can vouch for this on ntp version 4.2.4, not sure when it became available, but the command is

ntpq -p <hostname-or-ip>

Should show you the list of peers that the ntpd at the specified host is sync'd to, offset and jitter, and what not. The hostname is optional if you want status of ntpd on the current host. If you build ntpd from source, ntpq should be included, along with a bunch of other utilities.