You get a neuro-psychological assessment done by a neuro-psychologist. They can be quite expensive, up to $4000 depending on the neuro-psych and the battery of tests he uses.

They are valuable at diagnosing the conditions of your brain. It can show memory difficulties, executive function problems, etc. With this information, you can start working at learning ways to work-around the dysfunctions you have, rather than constantly questioning your own sanity.

The Halstead-Reitan battery is the most commonly used battery of tests.