Track "Accuracy of Commit" Metric
And review in the retrospective. The goal of the metric is to determine if actual velocity got within 20% of committed velocity, Sprint over Sprint.
Teams that track and talk about this metric find that they can improve their stabilize the commitments. When a team does this the Product Owner can trust information such as the Story Points and the Velocity and that means they can use the data for improved planning. In other words we would to establish a “commitment-based plan”.