Disconnect the battery
Locate the starter solenoid
Inspect all wiring and terminals for looseness, corrosion, or damage
Reconnect the battery
Set a multimeter to DC voltage
Check for battery voltage at the solenoid input terminal
Have someone turn the key to the start position
Check for voltage at the solenoid trigger terminal
Listen for a click from the solenoid
Check for voltage at the starter output terminal while cranking
Test continuity across the solenoid terminals if removed
Test resistance of the solenoid coil with a multimeter
Replace the solenoid if it fails any test