Lab, imaging and bio. Ct imaging can help the environment, both location and size. Blood tests are used to help clarify its presence, and look for associations with myasthenia gravis. Testing that can done and include test to see for associations to myasthenia gravis, that include antibodies to acetylcholine receptor, blood counts, chemical panels looking at liver and kidney, immune evaluation of the cells inside.
Imaging. The only way to know if you have a thymoma is to get tissue. There are often detected on imaging like a ct scan. When found, they are often resected either through a sternotomy or minimally invasively by thoracoscopy or robot. Sometime you can biopsy these, but their location often make them not accessible to a needle biopsy.
CT biopsy. Or direct to surgery if suspicion is high enough.