This is dependent on how an individual advertiser views incremental sales. This could be a percentage of new customer sales, higher AOV or increased conversion rates.
The answer here is to:
Ensure your program is set up to optimally track and report all the most important incremental KPIs
Test and review.
One way to do this would be to look at performance metrics before partners have started promoting your brand and then measure performance against a like for like period after they are live.