A server monitoring tool. :-)
There are many good options. I like StatusCake because they have a free tier which does everything you'd need for monitoring Tor relays. You can have it verify the fingerprint of your relay (via the information exposed by your DirPort) at the same time it checks if it's running.
There are a lot of other choices for hosted monitoring. Pingdom is a popular commercial choice.
There's also a wide variety of open source tools if you want to do it yourself and not rely on a third party. Icinga is a popular open source choice.