Does an employer have to disclose what kind of drug test or what drugs they want tested?

Not to the test subject, they don't.

Most employers who drug test will specify a "standard" test for them. Quite often it's just the "SAMSHA-5" drug test--pot, cocaine, PCP, opiates and amphetamines/methamphetamines. But the employer can specify other drugs--barbiturates and benzodiazepines are commonly tested for. They can also ask for specific tests--they can ask for prescription drug abuse screening, tobacco, alcohol, whatever they want. I think there are some protected classes--they can't ask that you be tested for AZT, which is an AIDS drug, or Trobicin, which cures gonorrhea.

But yes, they can send you in to be tested for any drug they want. There are even drug tests that will look for hundreds of drugs at once--they cost several hundred dollars so they're used for stuff like checking people into rehab (where you want to see EVERYTHING the patient is on) or trying to treat a drug OD when you don't know what the patient took.