Ask about disaster recovery, backups, penetration testing, what to do if a database/webserver breach occurs, or any form of generic networking/routing.
Not sysadmin specifically, but when I was working as a computer technician, questions we would ask to scope out the skills of people were along the lines of:
Q: "Something you have no experience with has broken and won't do a basic function, how will you go about fixing it?"
A: Research, ask someone else etc.
Q: "X is missing their password for a system we have no admin rights on. How are you going to fix this?"
A: Recovery Tools. Format device and set it up again. Use JS/Developer tools to get it in plain text from the browser.
