One of the simplest ways to measure reachability of your websites is by using Synthetics. These are automated robots that will do all the checking and reporting for you, everyone can understand them, anyone can deploy them.
Synthetic have advanced a lot, the real value comes when executing user journeys; and to do that you need to record and replay scripts against your portals. Logon, search for a product, add to cart, add shipping address, add voucher code, add payment options....you get the idea. The outcome is that you have completely understanding of what is working and what is not.
By clicking and registering here "https://grafana.net/" you are able to register to get a few very cool tools that you can use for free. Its based around you being able to quickly deploy synthetics to monitor your websites, but sat behind this is a brilliant set of visualization technology called "Grafana".
By registering for this cloud offering, everything is setup for you. All you need to do is add some url's to monitor, select the frequency, the locations you want to test from (many options), and away you go. Within 10 minutes you have something like this:
As this is a free version, it will show you the basics, enough to learn how to use Grafana and deployment of synthetics. There are limits on the number of alerts you can send. But, the best part is that you can use Grafana in the cloud to play with...add some new Plugins (https://grafana.com/grafana/plugins/) to explore all of your data visually, create new dashboards.
Like always, if you need help deploying Grafana in your enterprise, just reach out and we can hop on a call and help.