In the template that you want to override, you can use {$DesignKeys:used|attribute(show)} to see the available override conditions.
I recently added an enhancement request to get this mentioned in the documentation somewhere: http://issues.ez.no/12094
As for the variables available in the template, you can use this extension (didn't test it yet but if it does what it says then it's what you're looking for): http://projects.ez.no/show_variables