The insulator should be clean - off white to tan, maybe with some tiny reddish brown specs. If the insulators are black that is an indication of a rich mixture or too cold a plug or short trip driving that doesn't allow the plug insulator to warm up sufficiently to burn off deposits, but from your driving habit description this doesn't sound like the problem.
As suggested a check of O2 sensor performance is a good place to start. If you live in an area that requires emission testing, a mixture rich enough to leave carbon deposits on the insulators will likely bust an emission test. If the car is running okay you could wait 'till the next required test, or have a "training test" done to see how it's performing.