Any good mechanic knows the value of "reading" spark plugs - just removing the plugs and looking at them tells many things about an engine's condition.
For best performance and mileage it's a good idea to remove your spark plugs periodically between tuneups, checking the gap and, if necessary, resetting it. When you do this, take a couple of minutes to read the plugs.
The porcelain insulator around the center electrode - that's what you're interested in. It should be tan or grayish-tan. If it is, the cylinder you removed that plug from is getting the proper amount of fuel-air mix and the plug is firing adequately. Easy.
The hard part is diagnosing what's wrong if the porcelain insulator is not the right color. Your best bet is to take it down to your local garage and have it checked out.
If the porcelain insulator of each spark plug is dark in color and wet with gas, the spark plug in each combustion chamber may be getting too much gas and not enough air. A couple of possible causes: a stuck automatic choke and/or a very dirty air cleaner. In any case, you're wasting gas and your engine is not performing as its maximum - which you probably are already aware of if your plugs look like this.
If the insulator of only one plug is wet with gas and the insulators of the rest are all tan or grayish-tan, suspect a misfiring plug, since a stuck choke or dirty air cleaner since affects all the plugs, not just one. Suspect a bad plug or faulty spark plug wire.
If the plug is wet with gas, replace that spark plug wire with a new one. If the plug is not wet with gas, the spark plug that was previously in that cylinder was bad. Remove that plug and replace it with a new one. (On new cars with catalytic converter, which can destroy its effectiveness in reducing emissions.)
If the porcelain insulator of one or more plugs is dark in color and covered with oil, this indicates an oil control problem. If all the plugs are like this, and your car has a lot of miles on it, your pistons are probably shot and it's overhaul time. However, if you don't have the money for an overhaul right now, or you just want to get a little more use out of the car before you get rid of it, you may get temporary relief by going to a spark plug of a hotter heat range (check with your mechanic on this before doing it).
In general, if the porcelain is any color other than tan or grayish-tan, if the porcelain is cracked, blistered, chipped and if the electrodes are severely eroded, have a mechanic check your engine out. This way you may prevent a costly problem from developing.