Start by testing if the starter gets power on the start signal wire each time the key is turned. If not, the ignition switch is most likely culprit. If it does, check for battery voltage on the main power lead. A voltage drop test would be best for checking cable/connection integrity. A quick and dirty test for proper ground would be a jumper cable on the engine block to the battery negative. If it still fails, the starter probably has a bad spot in it.