Depends on what you mean by "bad". You can pull them and see if they are soaked in oil from running to rich and that will eventually make them go bad. You can test the resistance in them or go to autozone or advanced and buy a gap tool. Then see if you need them regapped if they are capable of being regapped. You can see if the electrode on the end is still good and not burnt up. Just somethings to look for. Hope this helps