Here are some good questions:
1. Ask about their love for your product, because if they don’t love it how can they make others love it.
2. Have they promoted a similar product or a competitor
3. How much do they usually charge per post (and why)
4. Ask to see their best engagement posts and then ask why they were successful and can they repeat the same success with your post and why.
5. Ask what the optimal time and days have been for their most engaged posts.
6. Do they delete the post the day later or keep them forever?
7. Last, ask them if you can keep the creatives from their sponsored posts that you pay for.
I strongly recommend using a contract for any sponsorship to protect yourself and your business, such as my Brand-Influencer Contract
SMI* = Social Media Influencers