In the treatment of endometriosis surveillance is always important. Once you've begun medical therapy for endometriosis, there should be surveillance of how you are doing, that your symptoms continue to be controlled, and that your priorities are being met. If it's pain control, and as time goes by the severity of the pain is not worsening, then the current medical modality that you're using is continuing to work. At the point that it is no longer effective, then there can always be a change in the medication therapy. For instance, if you start with birth control pill and that's no longer controlling your pain, you can try a continuous birth control pill.
If that's not working, you can always switch to perhaps an injectable progestational agent, such as Depo-Provera. If you get to a point where that perhaps is not as effective, then you could switch to something like a GnRH agonist, which is an injectable medication that suppresses hormone production and thereby suppresses the endometriosis.
When medications are failing, whether you've tried one medication or multiple medications, the definitive gold standard for diagnosis of endometriosis is to identify it visually, but also biopsying and having histopathology confirmation that you are dealing with endometrial glands and stroma outside the uterus. If medication is not working, then your doctor may consider a diagnostic evaluation for endometriosis. In that same process, areas of endometriosis that are found can be surgically removed.