When we were exploring plugin development earlier, we learned where to find and how to fetch the source code for any given Jenkins plugin.
The full source code for most plugins can be quickly and easily downloaded from GitHub and then built on your local machine. In many cases, this also includes Unit tests, which are bundled with the source code and can be found in the expected (by Maven convention) location of src/test. Examining a selection of popular plugins would provide you with useful information and a great starting point to write your own test cases.
The Maven test target will execute all of the tests and produce a summary of the outcome by detailing all the usual statistics such as the number of tests run along with how many failures and errors there were and the number of skipped tests.
To demonstrate this process, we will take a look at the very popular Green Balls plugin, which simply replaces the standard blue balls in Jenkins with green ones.