When your starter solenoid is bad your vehicle will usually do nothing when you turn the key.
So when you go to turn the ignition key to start the car and nothing happens then it could be the starter solenoid.
That is if you don't hear a clicking sound coming from the solenoid. If the solenoid is clicking then it's likely that something else is wrong such as low battery, bad battery cable connections, or lose battery cable connections.
If the battery is fully charged and all the connections to the battery and starter are good and you don't hear any clicking noises when trying to start the car then the solenoid is likely bad.
If the solenoid is on the side of the vehicle near the fender then it's cheap and easy to replace but if it's the solenoid on the starter then it's best to just replace the starter along with the solenoid since the starter will have to come off anyway.