Working with your doctor and your symptoms you can know based off of research and if your symptoms match up. For instance, painful periods are not normal and should be a red flag that something more is going on. Other common symptoms may help to know, but talking with your doctor about all of your symptoms (including ones that are more intimate) can help you know if you have endometriosis. The true way of knowing is by getting a diagnosis which takes an excision surgery. So, talking with your doctor is really important.