I always ask for examples of other companies they’ve worked with similar to mine and if I can speak to any as a reference. You can also ask for a list of recent roles they’ve filled. For me, I find the role they filled often aren’t as important as the kind of companies they have worked for.
I also would share why the role has been hard to fill and ask them what kind of tactics they will use to address those issues.
Finally, I suggest being very clear about your expectations of how the process will run, what they are responsible for, and how often / where in the process you expect them to fill in.
Also, I just spoke this week to a super interesting new recruiting firm that uses AI to speed up the process and charges a flat fee – https://www.uncommon.co/ if you’re interested. (I don’t have any affiliation with them I just liked their model and they are a startup so really focused on satisfying first rounds of customers).