NagiosXI and other monitoring tools have ways to determine if your SSL certificates on say a web server are nearing expiry, but what if you have software that uses a certificate but that certificate is not directly poll-able over the standard HTTPS checks.
The script which can be found here, is a passive check, so you would put the script on the server you wish to monitor, then configure it to run within a scheduled task perhaps once a day. You would then configure a host and service check within Nagios (and set it as a Passive check), the script would then report in once a day to tell you the status of the certificates on the server.