The distributor cap doesn't sound like such an important part, but it is integral to the performance of your car's engine. If the distributor cap is faulty then the ignition sequence inside your engine will be out of order. Here are a few tell-tale signs that your distributor cap needs to be replaced.
If you're idling at a stop sign or red light and your car seems to be shaking more than normal, that could be a sign that the rotor inside the distributor cap is letting ignition electrical pulses go to ground, and therefore the engine misfires.
Starting in the Cold
If you live in a cold weather area, unfortunately your distributor cap will be more likely to break than if you live in warm weather. This is because the cap is covered in plastic and when it goes from cold to hot instantaneously, it can crack and break. If you notice that your car has trouble starting on a regular basis, check the distributor cap for cracks. If you see one, you need to have the cap replaced. Even if there aren't cracks in the cap, there may be corrosive materials on the inside of the cap, which would also render it ineffective.
Stalling and Backfiring
If your car starts okay but acts up once you're on the road, stalling and/or backfiring, this could be because your distributor cap has a fault. Moisture trapped inside distributor cap can mis-direct the ignition electrical pulses to the wrong terminal in cap causing severe misfiring.
If you notice that your revolutions per minute are all over the place, it could be because the connection between the distributor cap and the distributor wires has gone bad. Again, check the cap for cracks or evidence of burning, singeing, or corrosion.
Trouble Turning Over
Sometimes people notice that along with their car stalling, their engine seems to turn over some times and not others. You will probably realize that it will turn over when the engine is cold, but not when it is hot. This can definitely mean that there is something wrong with the distributor cap, so have it checked out. If the cap seems okay, it might be another component in the distributor that is throwing off the ignition sequence.