An orthopedic surgeon can help diagnose the problem. History, exam, and imaging studies are the key tools used. Ultrasound can be used to assess tendon thickness and structure. Any irregular areas will also show up.
The results of this evaluation may point to the need to conduct further tests. A biopsy to examine the tissue in question under a microscope may be done.
The pain you described suggest your symptoms are made worse by movements and activity that loads the Achilles' tendon. This is common with a condition known as tendinopathy. Although you didn't mention your age, many middle-aged recreational athletes are affected by Achilles' tendinopathy.