How do I find out what dwarves, if any, have a given job active in their labor preferences so I can either remove other preferences that are taking up their time or change another dwarf's preferences?
tzenes answer is strictly correct. There is however no way in the game to see a list of all dwarves with a specific labor enabled (ie: mining). You must iterate through all your dwarves to check.
However, a custom utility called Dwarf Therapist has been written to allow you to do exactly what you want to do.