Amount of voting
Voting is the cornerstone of the moderation and the very first privilege. People should be voting often. And ideally, not just upvotes. Someone who doesn't use the moderation tools they have now, why should they be given more moderation tools?
Like it or not, moderators will at least need to read and answer questions on meta from time to time. Someone who never touches meta may have a harder time explaining past stances and demonstration of fairness in thought.
- Rep you would have if you had rep on meta
- Meta badges
- Meta questions/answers
Math.SE is just one stack exchange amongst many. How much familiarity elsewhere on Stack Exchange does the person have? Will they know when to migrate something to Physics.SE if they have never visited there?
- Active registrations
- Total reputation
Badges give a broad spectrum of what the person has done on the site that are applicable to participation and activity that a moderator would likely need to do.
How active is the person now in various moderation activities?
- Deputy Raised 80 helpful flags
- Civic Duty Voted 300 or more times
- Strunk & White Edited 80 posts
- Electorate Voted on 600 questions and 25% or more of total votes are on questions
- Sportsmanship Up voted 100 answers on questions where an answer of yours has a positive score
- Copy Editor Edited 500 posts (excluding own or deleted posts and tag edits)
How active is the person? on meta? Do they interact well with the rest of the community?
- Convention 10 posts with score of 2 on meta
- Pundit Left 10 comments with score of 5 or more
- Outspoken Posted 10 messages in chat that were starred by 10 different users
While not everyone has all of these things, you may wish consider them as an indication of how active the person will be as a moderator doing these things. If a person hasn't raised more than a few flags, do they have an idea of how to handle the flags? If the person hasn't done any reviews, do they have an idea of what "Not An Answer" means when it shows up as a mod?
And while it's completely possible that someone who is familiar with moderation tools from being a moderator pro-term on a beta site, it is likely important that the person be aware of what Math.SE is - it's the second largest stack exchange on the network after Stack Overflow in terms of raw numbers of questions and questions per day. This presents challenges that someone who only has a handful of questions or answers or flags or votes may not be fully aware of.
The people you elect are the ones that are going to be handling your flags and arbitrating your disputes. If the person hasn't flagged spam when it is seen, helped mark questions as duplicate where appropriate, down voted a poor question or answer, did a low quality review, or suggested more than a few edits... what are they going to do as a moderator where all of these actions are binding?