In my experience, the selenoid seems to start going bad first. It will usually fail to start or just click and nothing else, particularly when the engine is hot. Once it cools off, the starter will work again. In worse cases, I've had to tap the starter with a hammer to get it going. But you'll only get a few of these instances where she'll crank again. After that it simply won't work anymore. Anyhow, here's some info I found . .
A defective starter either fails to engage or cranks too slowly to engage the engine electronics. Engagement failures fall into three varieties. The first failure scenario involves a "zero engagement" or "no-clicking" activation of the starter solenoid. In today’s vehicles, a condition like this can be caused by an unintentional activation of a vehicle anti-theft system. In other cases, a defective ignition switch, starter relay or loose wiring connection can cause the problem.
The most accurate way to test these components is to connect a DVOM to the primary wire connection on the starter solenoid. Using the min./max. feature found on most professional DVOMs or multimeters, record the voltage at the solenoid terminal when turning the ignition switch to the "crank" position. If battery voltage is recorded, the above components should be considered in good condition.
If the starter clicks, but doesn’t engage, the problem is usually caused by a faulty starter solenoid that’s not engaging the starter drive gear. In most cases, replacing starter solenoids is not a cost-effective procedure; replacing the solenoid by itself is very expensive compared to replacing the defective starter with a remanufactured unit.
The second failure scenario occurs when the starter motor is running but fails to engage the flywheel ring gear or when the engagement is rough and noisy. In most of these cases, the overrun or one-way roller clutch on the starter drive gear is worn or sticking. In the remainder of these cases, the flywheel ring gear itself is either worn, or has broken or missing teeth.
The third failure scenario is a low-cranking speed issue caused by worn shaft bushings or a seized engine. In either case, the starter’s amperage draw will be unusually high for the application. Also, when starter current draw is high, remember that battery voltage will often drop below 9.6 volts. This is enough to affect system electronics which, in turn, will affect fuel pump operation.