The distributor cap is a vital part of your vehicle, as it is ultimately this device that sends signals to each of the spark plugs, which in turn ignites your engine. If the distributor cap is failing, you may get severely weakened performance or a car that won't start up at all.
Fortunately, checking your distributor cap is not too difficult. This part should be very easy to spot, as all the spark plug wires lead directly to the cap, which is typically located in the rear of the engine. When dealing with spark plugs, you have to be careful not to get any wires crossed. Mark each piece with some masking tape and assign each wire and port on the distributor cap a number. This will help ensure that no mix-ups occur when you detach the cap and reattach it later.
Unplug each of the wires, then use a screwdriver to remove the distributor cap. Looking underneath the cap, you should see small electrodes where each of the spark plug wires connects to the cap. These are extremely important, as these are the contacts that distribute the spark signal to the wires. You should be able to visibly see if these electrodes are burned or oxidized. If so, you'll need to replace the distributor cap.
Another common problem is inside the cap itself. If you see black streaks on the wall, then it likely means that the sparks are misfiring and causing minor burns. Not only will you need to replace the cap in this scenario, but the spark plug wires as well.
One last thing you can check involves the use of an ohmmeter, which is available at many automotive parts stores. Attach one lead of the device to the exterior, specifically where the ignition coil connects to the cap. The other lead should be placed on the inside of the cap, where the ignition coil comes through the other side. An ohmmeter measures resistance, and in this case you want a reading of zero, or very close to it. If not, then the spark signals are not transmitting cleanly from the ignition through to the spark plugs, and you'll need a new cap.
Fortunately, replacing the cap is simple as well, especially if you followed the advice to mark each spark plug. This way, you can connect the new cap to the wires exactly as the last one was and get your car running well again.