This is not productive under any working conditions, and cause an almost unthinkable number of problems at your work place. A person bringing illegal drugs to the work place could bring unwanted legal action against your business, and everything economically would just snowball from there, and ultimately cause even more problems than one could originally expect.
If an employer is thinking a person may be taking drugs, the easiest and safest way to be sure is to simply confront the person about it. Sneaking around and trying to find out isn't going to help the problem, you need to be up front and direct. If that doesn't work, that you can ask them to submit to a drug test, but only if you are certain they are under the influence and are using drugs.
Drug abuse also does not just affect the person taking the drugs, it tends to have a ripple effect that can spread beyond a person's work or family life. Everyone person a drug user is it contact with can be affected by them using drugs, and it can cause of nasty chain of events that nobody wants to see happen. Drugs can change who a person is, and can make them interact harshly towards people.