I was let go from an employer in the past due to a dirty U.A. Im ready to go back into the work force without the dirty U.A. I was wondering what my past employer can tell my hopeful future employer?

They can tell them that you worked there, your pay rate, what you did, if you were a good employee, and why you left or got fired and if they would rehire.

So yes they can tell them about the dirty UA, most likely they will not, but there is no reason they can't

If the potential employer calls for a reference they can say nothing at all or anything truthful. Frequently they will give short answers and avoid getting into why you left or if you were fired just because they may not want to get involved in any potential law suit. That being said, they can say you failed a drug test and were terminated.