How does one respond to an ADD/ADHD diagnosis of one's self or one's child? I read:


  Is ADHD even a thing? 
  
  This kind of question can be offensive to some. The same kind of thing
  can be applied to other conditions:
  
  Is depression even a thing? Or are they just lazy? Is gender disphoria
  even a thing? Or have they just sexually confused? Is homosexuality
  even a thing? Or are they just a deviant? Is otherkin even a thing? Or
  are the just a teenager looking for a unique identity?


Apparently, there is much controversy over ADHD, according to Wiki (not enough reputation)

Part 1:

What is a diagnosed person to do? How does one evaluate methods used by a doctor? How does one evaluate if prescribed medicine and dosage is right or at least not severely wrong?

I'm guessing initial prescription, if any, is going to be experimental. From there we see if dosage needs to increase or decrease or if medicine needs to be changed or stopped. How then should one evaluate a doctor's decision?

Part 2:

Why not controversy over depression? Bipolar? Schizophrenia?

Some psychologists seem to believe ADD/ADHD is a myth. Is there any truth to such?

The blog also points out how homosexuality was wrongly considered a disorder. How do we know ADD/ADHD won't turn out like that?
There actually is the same kind of controversy over depression. The French documentary Dépression, une épidémie mondiale nicely summarizes how a well-meant change in diagnostic criteria (from DSM-IV to DSM-5) backfired and the pharmacoceutical industry now use this to diagnose every sadness or exhaustion as depression with the intention to sell drugs and make money.
There has also been controversy over schizophrenia for at least two hundred years. Other mental disorders are equally highly controversial. It is just that ADHD is a &quot;fashion disorder&quot; today and therefore covered by the media, which gives laypeople the impression that only this disorder is controversial.
As for ADHD, we live in a society that favours certain personality traits and disfavours others, and those kids that don't sit still very well, need a higher input speed, or are simply socially maladjusted, are often diagnosed with ADHD, although they are perfectly healthy and normal and would only need a different kind of learning environment or better familial support.
On the other hand, this does not mean that ADHD does not exist. If the diagnosis was given by an expert after careful investigation, then the patient will certainly profit from therapy. But the therapy cannot be deduced from the diagnosis alone. It must take into consideration the etiology (what caused the disorder?) and the circumstances (how does the patient live?), as well as the personality of the patient. Because the same symptoms of the same disorder can have widely varying causes, and you want your therapy to address those causes and not just ease the symptoms.
Therefore a meaningful diagnosis will include an analysis of the cause. ADHD can be genetic, caused by harmful substances, or psycho-social. Therapy will differ in each case.
If you want to evaluate a diagnosis and recommended therapy, I would recommend you go see another expert. Evaluating them yourself would require you study medicine and psychology to an extent probably not feasible. Be extremely careful of reading stuff on the net! Much of what you find there is published by companies who are in search of patients to sell medication to, or by laypeople.
I don't know where you live, and probably don't know how you can find trusted experts in your country. In Germany, where I live, the best path to take would be to approach therapeutic organizations or even your health insurance and ask them for whom you should turn to.
Generally, psychiatrists appear to be more likely to prescribe medication, while psychologists seem to prefer non-medical therapy. The best therapy for many mental disorders is a combination of both, with medication used to alleviate the current symptoms, and psychotherapy working mid- to long-term on changing behavior (and personality) so that medication is no longer necessary or can at least be reduced. I know people with ADHD who live well without medication, simply by creating for themselves the environment that they can flourish in.
All the best.

tl;dr
Find a doctor you can trust, and trust them.