Brand engagement is the leading indicator of ROI over the long term. Engagement can be defined as any action taken in response to a post, including but not limited to liking, commenting, sharing, retweeting, or reacting. You can find these on the internet, and they're a great place to start when researching a possible influencer because they show the engagement of the influencer's followers.
The number of audiences who engaged with your content in some way (like liking, sharing, or commenting) is a good indicator of its success. The number of comments, likes, and shares divided by the total number of impressions equals the engagement rate.