running e.g. "rake test:units:rcov" gives you a nice textual overview and coverage percentage on the shell console.
However, even more interesting are the generated HTML pages. At the end of the console output a line starting with View the full results at file:///... points you to the graphical overview.
Clicking on one of the tested files in the index page gives you the detailed analysis, showing - Code reported as executed by Ruby - Lines considered as run by rcov, but not reported by Ruby (e.g comments) - Lines marked as not executed.
The last group even has a red background, thus giving you clear hints on where to improve your test cases.