How can I troubleshoot a faulty starter motor in my Audi 80?

I recently bought a used Audi 80 and the starter motor seems to be failing. It's been starting slowly and sometimes not at all. How can I diagnose and fix this issue?

First, check the connections on the starter motor to make sure they are secure. If they are loose, tighten them up and see if that improves the starting. If not, it may be a sign of a more serious issue and you should take the car to a mechanic.