Monitor your site
There are lots of monitoring sites available to check if your site is running the correct way. If you use these monitoring sites on a regular basis, a change in answer times or performance evaluation can show you immediately that something is going wrong. A few of the tools I personally use are
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Uptime Robot
A free monitoring tool which can monitor up to 50 sites. You can get real time alerts if your site goes down by configuring your account. There are a lot of possibilities for real time alerts, including (free) SMS, E-mail, Twitter messages, PushBullet and Slack. -
Pingdom
A monitoring tool which allows you to subscribe for free (if you know the link!) but with very limited features. Anyway it can give you quite a good idea about how your server is doing and you can configure it to alert you by email, by SMS (to be paid!), with a Twitter message or through the PingDom app for both iOS and Android.