Well what a coincidence that you ask this, Jan! Just today I had time on a long train journey and decided to write a Rewrite Analyzer plugin, one that parses your rewrite rules and highlights the query variables. You can test URLs right there and see what query variables will be set.
You can find it in the plugin repository, or just as Monkeyman Rewrite Analyzer when search for new plugins in your administration screen.
solved A tool to analyze rewrite rules? [closed]