One factor that keeps so many victims silent in the instance of workplace sexual harassment is the fear they will not be believed. Many harassers do not commit their offenses in easily captured media such as emails or text messages, instead engaging in sexual harassment in private places with no witnesses. This results in "he said, she said" situations where the burden of proof may be on the victim, making you extremely reluctant to come forward out of fear of being dismissed or even punished. So how can you prove you were harassed when you are your only witness?
Often you cannot, but that does not mean employers can dismiss you outright without hearing your case and, if warranted, conducting a formal investigation before making a decision based on the evidence at hand. The California Department of Fair Employment and Housing sets out a guide for employers when dealing with claims of harassment, particularly those without tangible proof. Rather than using the type of standard one might in court in which incontrovertible evidence is required, instead employers are inspected to conduct an investigation into the plausibility of the events you described to determine if they are more likely than not.
Factors taken into account may include other instances of reports against the same person by other employees, or others' observations of your own potentially distressed behavior on the day the harassment occurred. Your harasser may have a reputation even if they have not been reported, or may have said something to another employee about you to lend credibility to your claims of harassment. Employers may question you, but are advised not to interrogate you or make character judgments even when determining if there is motive to lie.
This post is intended for reference only and should not stand in for legal counsel.