TL;DR: They don’t.
Here’s a longer explanation.
There comes a time in most discrimination lawsuits when a defendant-employer will file a motion for summary judgment. By filing that motion, the movant is telling the court that there are no material facts in dispute and, if the court applies the well-settled law of, say, Title VII of the Civil Rights Act of 1964, to those facts, then the movant should win.
Often, however, the movant is overly ambitious because the plaintiff may say that “x” happened, and the defendant may say that “y” happened. The defendant may have ten witnesses