It would be similar to the challenge of hiring a janitor for a building, he would get to have all the keys, he can open any door, but the reason is because he needs them to do his job. Symmetrically one can think of this age old problem and look at ways trust is granted historically.
Although there's no clean-cut technical solution to this human problem, the fact that there's none shouldn't be a reason that we don't try any, an aggregation of imperfect solutions can give somewhat great results.
A model where trust is earned:
- Give less permissions to begin with
- Gradually increase permissions
- Put a honeypot and monitor what happens in the coming days
- If he reports it and doesn't use it for his advantage, that's a good start
Implement several levels of administrative powers:
- Level 1: Can modify lower tier of configuration files
- Level 2: Can modify slightly higher tier of configuration files
- Level 3: Can modify slightly higher tier of configuration files and OS settings
Always create an environment where total access by one person is not possible:
- Split systems in clusters
- Give cluster admin powers to different groups
- Minimum 2 groups
Use the Two-man rule when doing high-level core changes:
Trust and verify:
- Log everything
- Log monitoring and alerting
- Ensure all actions are distinguishable
Paperwork:
- Have them sign paperwork to have the legal system be able to help you by suing them if they hurt you gives more incentive not to do so