You can have it done by a psychologist or psychiatrist. This is what I have learned from others. I have not had the test done officially though, I did the online tests and the results did not surprise me, so I don't see the need to have an official certificate of aspergers since it is not considered a disability.
No, they won't tell anyone, there is patient client confidentiality. They can only disclose this if there is a court warrant if you committed a crime and they want your mental history.
I did all of the online test I could find. All of them say that the chances of me having Aspergers ranges from extremly high to "pff , you are definitely asperger, and probably a few other things too". I don't need or want a certificate. It's more of a curiousity thing, and I have an extreme need to get to know myself (which apparently is a symptom aswell)