See the relevant bits in federal law about asking questions of employees dealing with DEA-controlled substances.
Here is a direct link to the federal code pertaining to screening employees and more regarding DEA law:
> Section 1301.90: Employee screening procedures
> Section 1301.91: Employee responsibility to report drug diversion
> Section 1301.92: Illicit activities by employees
> Section 1301.93: Sources of information for employee checks