There is not enough space here to address all the nuances of reading plugs so the following is strictly an emergency guide.

Prior to the plug check, the car must to be fully up to temp. If you are bedding brakes or "running in" a new ring gear, this is not the time to get a plug check. For the test you need to run all "hot laps". Install a new set of plugs for your check. Now, hurry out and run 5 to 8 hot laps and on the last lap get a "clean cut." A clean cut is where you come out of a turn (usually turn 2 on an oval track), run down the straight and at a safe distance from the end (while at full throttle), hit the kill switch and clutch together and drop the throttle. At that point, put it in neutral and coast back to the pit area and pull the plugs for a reading.

Look at the bottom of the insulator core (yes, way down in the plug). There should be a visible "fuel ring" at the base of the core. If by chance you are using leaded gas, the ring should be tan to black. If its not this color, put in bigger jets and repeat the process with new plugs. Some unleaded gas will produce a fainter ring so be careful. When in doubt always start way rich and work toward lean.

The ground electrode color and nose of the core can tell you about timing. There may be a slight discoloration of the ground wire indicating a good temperature. If the wire has a lot of discoloration with more at the end (from excess heat) and/or the core nose appears glazed, you need to reduce the timing. If you spot any tiny aluminum specks with your magnifying glass, take out a bunch of timing cause you are in danger of blowing the motor.

Be careful! Plug reading is not just this simple. Interactions between cylinders, cooling, weather, elevation and many other things will all influence the plug's appearance. But, when in doubt "go fat and late."