If you are in the USA, an employer cannot ask you if you have a disability or make that a determination of employment. This works in theory but in practice, not so much because they can ask if you need accomdations or tell something is off in the interview.
You also can tell your employer if you wish.
In my case, only 2 of my employers ever knew that is had any for of Autism and in both cases, it was because I said something to the boss.