How do I troubleshoot a failing starter on an Audi A2?

I've been having trouble starting my Audi A2 lately and I suspect the starter may be failing. How can I troubleshoot this issue and potentially fix it on my own?

First, check the battery to ensure it has enough charge. If the battery is fine, then try tapping on the starter with a hammer while someone else attempts to start the car. If it starts, this could indicate a faulty starter. You may need to replace the starter if this continues to happen.