There are a few different ways that you can make sure to get notified if your web host experiences an outage.
Status pages and forums
First, make sure to subscribe to your web host’s status page and server forum if they have one. Most web hosts have a status page where they will post updates on issues that are affecting whatever server. There should be an option to subscribe to the status page so you will get an email if there is any updates to that page. Another thing that many web hosts will create a forum specifically for the server you are hosted on. The web host will typically post status updates, maintenance, upgrades, and outage info into that forum. You should subscribe to such a forum if your web host has made one available.
Uptime monitoring services
Second, there are a lot of different uptime monitoring services available out there. You should find either a free one or subscribe to one that charges money to notify you if your site is not reachable. The free uptime monitoring services will probably either ping your site every 5 to 10 minutes. If the ping is not successful then they will notify you via SMS or email. If you want more advance monitoring or on the minute monitor (meaning they try to reach your site every minute) it will cost some money. You can choose to use either the free services or pay for more advance uptime monitoring to keep you up to date if there is an outage with your web host.
Twitter and Facebook
Third, if you use Facebook or Twitter then you can follow or befriend your web host. If your web host is good at communicating then they will probably provide updates or status updates on any outage they may be experiencing.
Your host doesn’t want you to know
Fourth, not all web hosts will reveal or let their customers know that there is an outage. There could be a outage where the whole network is offline and the web host may say its an isolated or just a subset number of customers had an outage. This is why you want to use a third party service to monitor your site for any connectivity issues.