One good approach to this kind of risk is to provide documentary evidence that you applied the currently-accepted best practices for preventing attacks, detecting attacks when they occur, mitigating the impact of such attacks as slip through, finding the gaps in your prevention strategy, and closing those gaps.
In my company, this is done by contracting a reputable security consultant to periodically scan our systems for known vulnerabilities. They provide a written report of the results, and we in turn provide written attestations that we have reviewed the report and fixed the vulnerabilities detected. To close the loop, the security consultant does another scan, and provides another report documenting that the previous vulnerabilities have been fixed.
This is basic information security. In my opinion, Hillary failed at her job not when she got hacked (which we may never be able to prove), but when she did not take even these basic steps to prevent hacking.
I might give her a pass if she had used State's email system, due to bureaucratic inertia, etc. But she chose to set up her own system, beyond government oversight. That neither she nor her advisors thought to establish their own oversight, to a high standard commensurate with the importance and sensitivity of the office of Secretary of State, is unforgivable.
I don't need to prove that she got hacked to know that she screwed the pooch. I only have to prove that she made no serious effort to avoid getting hacked. And that evidence is in: She screwed the pooch.