Coil tester. Measure the distance and analyze the color of the spark to detect if the coil is in good condition. The ideal spark distance is usually around 2cm. If the spark is purplish blue, is perfectly aligned, and reaches
2cm in length, the coil is in good condition, on the other hand if the spark is reddish, curved, and does not reach 2cm in length, the coil must be replaced.