Self experimentation. If the doctor prescribes something for you, test numerically whether it helps you to improve.
If you suffer from allergies it makes sense to systemmatically check through self experimentation whether your condition improves by removing varies substances from your diet.
It doesn't hurt to use a symptom checker like http://symptoms.webmd.com/#./introView to get a list of more possible diagnoses.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.