2 Answers

Hope this helps :http://troubleshootmyvehicle.com/gm/3.8L/how-to-test-the-starter-motor-1

Do you mean the neutral safety switch? 

 Instructions
1
Use a service manual to locate the neutral safety switch in your vehicle. The placement may vary between vehicle models, but the testing procedure remains universal. The neutral safety switch will resemble a cigarette lighter, and is usually located near the shift mechanism. In column shift vehicles, the switch is threaded onto the side of the transmission, near the gear shifter.

2
Check the wiring. The switch has wires that connect the switch to the starter solenoid. The switch remains "open" when the transmission is in gear, which prevents electrical current from engaging the starter. When the ignition switch is moved to the "Start" position, this allows current to flow to the starter, as long as the transmission is in "Park" or "Neutral" gear. Refer to your service manual for the wiring schematic of your specific vehicle model, and detach the switch from the wiring connector.

3
Put the key in the ignition switch with the gear in the "Reverse" position. Do not turn on the engine. Check whether there is any power in the wiring harness. Use a 12-volt test light. If it shows an illuminated bulb then there is power and the circuit is working properly. Attach the wiring connector back to the neutral switch. If the test light does not illuminate, test all wires for loose or damaged connections, then retest. If no power is present, then replace the neutral safety switch.

Read more: How to Test a Neutral Safety Switch | eHow.com http://www.ehow.com/how_7563907_test-neutral-safety-switch.html#ixzz2PbGrELVm