How can I tell if I need a new starter?

If the battery is okay but the engine still cranks slowly or not at all, it is time to check the starting system. The starter could have failed, or excessive resistance in the wires and their connections can prevent the proper voltage from reaching the starter causing slow or no cranking.

Check for loose or dirty cable connections or for battery cables with wires that are too small in diameter (gauge). If the starter solenoid clicks but the starter does not operate, it is recommended to use a voltmeter to determine if the proper voltage is reaching the solenoid through the starter control circuit. If the solenoid is receiving the proper voltage and the starter does not crank, then the solenoid ground may be bad, the solenoid contacts may be damaged, or the starter is dead.

If all connections are clean and tight and the voltage drop exceeds 0.2 volts, the solenoid should be replaced. If everything else checks out as okay, measure the starter draw and amperage, if incorrect, replace the starter. If the solenoid is defective but attached to the starter motor, it may be best for you to replace the complete starter and solenoid assembly- there's a very good chance the starter will die very soon along with the solenoid. Why risk a comeback and more delay getting the vehicle up and running.