Use job skills test before hiring to make sure applicant is qualified

It happens all the time: A manager decides to take a chance by hiring a marginally qualified applicant. Then, days later—as the new employee struggles—it becomes clear she can’t do the job. Employers have little choice but to terminate the worker. And then the former employee feels like she has little choice but to sue for some form of discrimination. What's the best way to avoid those kinds of lawsuits?