Nagios can be seen as a automated test tool for systems, just as you would have automated tests for software projects. In test driven development (TDD), you write the tests first, and then use those tests to build up a software project that you can have confidence that it works. We can use this method to build up systems, or networks of systems. Plan out which services and processes should be running on your new systems, and then implement Nagios tests for every one. You can check the progress of your build by checking Nagios. I have been doing this at the BBC. It is a simple idea but one that seems to work.