File a police report. The good thing about computers is also the bad thing about computers, they have a history. If this person is using a restricted system (such as a court or police computer system for work) and is misusing it, the person may be subject to diciplniary action at work and also criminal charges. It would be easy for the employer to check her computer history to see what she has been doing. Save the email and show it when filing the police report. Good Luck!
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Emphasis on preserving the email. You might want to talk to a private investigator who has experience with such devices. You should also check their credentials as in credibility.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.
Your question is too vague to give a good answer because you do not describe what the person is doing with enough particularity. For example, I cannot tell whether the person is sending defammtory emails about you, is altering courthouse records to make it look as if you have pending criminal or civil charges, or something else. Also, I do not know what you are asking "How can I know for sure?" I can't tell if you mean how can Iknowwho is doing it, or if you wnat to know for sure what she's doing, etc.
If someone is publishing defamatory things about you, via email or otherwise, you could consult an attorney about suing the person.