The only 100% sure way to know is to remove the starter and had it bench tested at your local auto parts store.
try charging the battery before you replace the starter
Bad starter solenoid. Bad brushes more likely (worn out)
If your starter is bad you simply replace it with another one.
Typically, the first indication is the starter solenoid clicks but the starter does not spin. This indicates a dead or weak spot on the armature where the brushes must make good contact. Usually, trying to start the engine a couple more times will result in success. This could go on for a very long time before you actually need to replace the starter (I know). But, it is better to replace as soon as this happens a couple times, because you just don't know when it won't resuscitate itself someday. The other indication that a starter is going bad (or is bad) is if it pulls the battery way down when you try to start. This indicates a shorting of the armature to ground during rotation. Replace immediately.
Make sure the solonoid is bad first.. It is on top of the starter and the starter has to be removed to replace it.
Bad starter is easy: when you turn the car on provided your battery is good, you will hear a click in the engine compartment, but the car will not start.
You know the starter is going bad when you have a dragging starter.
AnswerCould be a bad starter drive - spring broken and not releasing or a bad starter solenoid not opening circuit See "Related Questions" below for instructions on how to remove/replace the starter.
Typically, when the solenoid fails the starter is right behind. If you know how you could replace the solenoid, put in brushes and bushings and the starter MIGHT be ok if nothing bad has happened to the armature.
the engine will turn over a few times before starting, if it starts, and it will feel sluggish
the bendix gear on the starter is stuck on the flywheel remove starter check bendix gear for wear if worn to bad replace starter
A bad starter might make a noise. The starter could also go bad with no noise or warning. However if the starter is making noise then there is a problem that could cause failure at any time