If you have a reasonable suspicion that a DOT-regulated safety-sensitive employee is using drugs, you are to send him for a Reasonable Suspicion drug test. Your determination to test must be based on specific, contemporaneous, articulable observations concerning the appearance, behavior, speech or body odors of the employee. The supervisor making the Reasonable-Suspicion determination must be trained in reasonable suspicion.
Conducting a “random” test without documentation that shows how the individual was selected could put your company at risk for legal action. See What exactly IS a “random” drug test?