Bad capacitors are a common problem for many electronic devices. For easy understanding, they dry out with use, this causes them to overheat venting their gas. Failed capacitors are usually very easy to spot even for the untrained eye. They have a distinctive, "tin can" look. The top of a new capacitor is totally flat with 3 small slits. These slits are vents. When a capacitor reaches the end of it's service, the pressure builds up and opens the vents. All you need to do to spot a expired capacitor is to look at the top of it to see if the vents are open, this will be apparent by the swelling of the top of the capacitor. The top of the capacitor will no longer be flat but will be slightly raised in the middle.
Hope this is useful.
Remember. High voltages are dangerous. Always respect electricity because it won't respect you.