This is easy. Since you have already established a good positive source, attach one end of the circuit tester to the positive end. Now touch the other end of the tester to the ground wire for this component. If it lights up you have a good ground and need to check the component further. If you don't get a light, it's time to clean contact points and check the ground path. Luckily, grounds aren't too bad to re-establish.
How To Test a Circuit Using a Test Light
By Matthew Wright, About.com
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Using a Test Light to Check a Ground
Testing for ground is the reverse of a voltage check.
photo by Matt Wright, 2008
Your test light circuit tester is great for checking for voltage, but it can also be used to check a ground circuit. If you know that a certain electrical component is getting juice on the positive side, you need to check to see if it has a good grounding point.
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