The best way to check if your distributor caps and distributor rotors are working properly is at night, because you can easily see the lights that they give off from the current. You should not touch them, though, because they have as much as 20,000 volts running through them.

(http://www.innerauto.com/images/partImages/distributor_cap1.jpg)  (http://www.innerauto.com/images/partImages/distributor_cap2.jpg)

Check out more tips on distributor cap (http://www.innerauto.com/Auto_Part/Distributor_Cap/).