Pay discrimination based on sex has long been illegal. Both the Equal Pay Act of 1963 (EPA) and Title VII of the Civil Rights Act of 1964 prohibit paying someone less for the same work because of gender. But it can be tough to prove, says labor and employment attorney Ryan D. Barack with Kwall, Showers, Barack & Chilson in Clearwater, Florida. Barack should know: He successfully represented former Citicorp service center manager Heidi Wilson in a gender-based wage discrimination case against Citicorp North America, a subsidiary of Citibank.

Wilson was paid $75,329 when she was promoted to manage a Tampa service center. However, her male predecessor was paid more than $129,000 the year before, according to a report in the Tampa Bay Times. In 2011, she was bypassed for another raise and objected. Shortly thereafter she was fired without severance and took the case to court. In 2012, Wilson won the case—and was awarded nearly $340,000 in back pay.

Unfortunately, Wilson’s case is unusual in its clarity, says Barack. Her job was identical to the one held by her predecessor, so once the wage disparity was revealed, the inequity was indisputable. It’s rarely so easy to prove, he says.

Proving Unequal Pay

In FY 2015, the Equal Employment Opportunity Commission (EEOC) received 973 equal wage discrimination complaints, a number that’s held relatively steady since 1997. Yet just a fraction of these complaints go beyond the initial filing. Last year, the 64% percent of complaints received by the EEOC were found to have “no reasonable cause” for action.

Part of the reason these cases can be so difficult to prove, says Debra Lawrence, a regional attorney in the EEOC Philadelphia District Office, is the four affirmative defenses built into the Equal Pay Act. If there’s a disparity in pay, an employer must prove that it’s justified by:

• A seniority system
• A merit system
• A pay system based on quantity or quality of output
• Any other factor other than sex

While the first three factors are pretty straightforward, that last “catch-all” category is where employers get creative, says Lawrence. They may say the higher paid employee has more experience or training, or that he was simply a better negotiator. In addition, just obtaining the salary information to show the pay differential can be difficult—especially when employees are discouraged from discussing what they’re paid.

There is, however, one unique factor in equal pay cases that actually works in the complainant’s favor. Unlike most discrimination actions, the Equal Pay Act intent-neutral. In other words, it doesn’t matter whether you meant to pay an employee less because of gender—the fact that you did it is enough. Once the disparity is established, the burden of proof that the reason was a legitimate one is on the employer, who should be able to show that any policy that resulted in disproportionate pay is in place throughout the organization, says Lawrence.

Here’s a look at some other successful cases:

In a $45,000 settlement announced by the EEOC in March 2016 with NFI RoadRail LLC and NFI Industries, Inc., a New Jersey-based business that provides logistics, transportation and warehouse services, a manager discovered that she was being paid less than her male counterparts when she came across a former co-worker’s pay stub.

Wendie Dreves’ 2011 case started as a gender bias suit. She alleged that her employer, Hudson Group Retail, LLC, did not afford her the same accommodations as her male replacement. Her attorney, Burlington, Vermont-based John Franco, says Dreves covered for hourly workers who were not replaced, picking up the slack without additional compensation—something not required of her replacement. But in discovery, Franco says they learned that her replacement was hired at roughly $4,000 more than Dreves was paid, despite the fact that she’d worked at the company for 10 years. A summary judgement was issued in favor of her pay-related complaint.

Not all pay discrimination cases focus on individuals. An EEOC complaint against Market Burgers, LLC, which operates a Philadelphia Checkers franchise, alleged that the franchise was paying female shift managers, cashiers and sandwich-makers less than their male counterparts and also scheduling them for fewer hours. In April 2014, the EEOC announced a $100,000 settlement with the franchise owner.

Taking Action

If you think you’ve been the victim of wage discrimination, you can file a complaint with your local EEOC office or contact an attorney directly. You are permitted to do both, though Evelyn Murphy, founder of the nonprofit WAGE Project, suggests starting with the EEOC—or your state-level equal employment agency—since they can offer free advice before you decide to shell out for a private lawyer.

You’ll want to create a paper trail. Document as much as you can (legally) in terms of the disparities. Lawrence recommends including posted or published job descriptions, pay data, length of employment, and other relevant details.

While women are generally protected from retaliation once they bring a claim, the process can be harrowing, says Murphy. Allegations of wage discrimination may be met with an employer and its counsel attempting to discredit the employee’s competence or quality of work. She’s seen women “losing their own self-confidence after pretty brutal attacks in court,” she says.

Keep in mind that proving job equivalency tends to get difficult as you move up the career ladder—especially in white collar jobs. “When you have somebody in a factory making widgets, [proving] the equality is going to be easier,” says Lawrence. “When you have professors of different subject matters within the same department of a university, that’s going to be harder.”

Barack says one of the key ways to fight back is to create transparency about who is being paid what—even when employers try to forbid it. There is power in information.

“The short answer is that all employees need to break the stigma about discussing their pay,” he says. “Although many employers have policies—both written and unwritten—about [not] discussing how much you earn, those policies are illegal under federal law.”