How can I diagnose and fix a faulty starter in my Audi 80?

I've been having trouble starting my Audi 80, and I suspect it may be due to a faulty starter. What steps can I take to diagnose the issue and fix it?

First, check the battery and starter connections for corrosion or loose connections. If those are fine, use a multimeter to test the voltage at the starter solenoid while attempting to start the car. If the voltage is low, you may need to replace the starter.