Velocity & Average Velocity
If you really want to know the health of scrum implementation, then ask the developers about the velocity of the previous sprint?. If all of them gives you the same number, then it is a good implementation of scrum.
Velocity = Sum of the story points completed in the last sprint
For example, for sprint#1 we planned 10 stories to be developed. Let us assume that the sum of the story points of these 10 stories put together is 19. At the end of the sprint#1 the team could complete only 17 story points. In this case;
Planned velocity for sprint#1 = 19
Achieved velocity = 17
So, velocity is the story points completed in the previous sprint. That is the reflection of the capability of the team.
After completing several sprints, the team will be in a position to calculate the average velocity of the sprints completed. The number of story points to be completed (from the product backlog) divided by the average velocity gives the number of sprints required to complete the project.