I assume you are talking about wire wheels. If so, there is a 5/16 - 24 stud on the end of the cap that you can thread a nut onto. Once the nut is in place, hook something behind it and pull the dust cap out. If it is reluctant to come out with hand pulling, grasp the end of your hooked implement with some vice grips and use a screwdriver to lever it out. This is a quick and dirty way to get it out. You may want to make up a tool that can be threaded on the stud and attached to a slide hammer or some other way to means to pull the cap out.
Good luck,