What are best practices for occasional pot smokers when confronted with corporate drug tests?

Someone I know occasionally smokes weed and is looking for a new job. Some of the jobs she’s looking at require drug tests. (We’re in Chicago, so it’s not legal here.)

She actually had to take a drug test for her current job—as an occasional pot smoker—and got hired without any problems. (We’re assuming they just didn’t care that she smoked.)

Should she take the tests and hope for the best? Or should she admit at an interview that she has occasionally smoked? And what is the likelihood that corporate HR in a big city job will even care if she tests positive for marijuana?