There are (at least) two ways to ask questions of candidates: via the meta survey, and via a town-hall chat (if your site is doing one of those). In addition, reviews are public information so anybody can look at how any candidate handled reviews.
Meta survey: On one recent election the community was surveyed to collect candidate questions, and then a com-team member posted a questionnaire that all candidates were invited to answer. If concerns about flagging are important on your site, you can work to include a question about it on such a questionnaire.
Chat: It appears from that election that the questionnaire has deprecated the town-hall chat. However, nothing precludes a site from setting up its own chat, either live and real-time like that or open over a period of time. Again, given a venue, you can then raise whatever questions are most important on your site.
Reviews: Almost everything a person does on Stack Exchange (votes and flags excluded) is public information available on the "activity" tab of the user profile. You can filter this view to look just at edits, or reviews, or comments. This doesn't tell you how the person has handled flags (if he's 10k and thus can handle some), but it gives you a sense of how he performs other site moderation. Any individual can inspect this data for any candidate(s), and perhaps a community-minded volunteer would compile it into an easier-to-review form for everybody else.