You would have to use monitoring software or a monitoring service to provide you with the actual stats as to uptime for your servers. I am looking at Netstatus right now. Not a perfect solution (far from it) but gives the basic information.
Also some hosts also use a service such as Alertra.com to monitor their servers.
Our customers can look at our public hostingassured.com stats but it may not be the most reliable as it doesn't seem to poll the servers as often as it should. We simply keep track of downtime when we receive reports, our own monitoring systems go off, and/or our customers can file technical supports with time lost due to suspected network or server problems. Our admins are paged instantly and automatically whenever a server is not responding. We have a system of scripts that I have posted here in the past designed to test our servers from many different networks.
Hosting assured checks once every hour. I most certainly would never consider using that to substantiate payment on an SLA. You could have 3-2 minute reboots in a month and show 3 hours of downtime!
I would suggest you use a reliable 3rd party remote monitoring company like Alertra. A script that you have installed on your server can be modified. You cannot alter the results of Alertra or companies like them. Make sure monitoring is done at least every 5 minutes and make the results available to your clients.
If you experience downtime the very last thing you will want is a dispute on the amount of downtime you have. Your clients are already going to be upset about it, make sure the information is crystal clear to avoid any negative escalations....