Starting today,you can monitor the capacity and performance metrics of your Spot fleets via CloudWatch. You can view metrics such as total desired, fulfilled and currently launching capacity of a fleet, or in addition to monitoring aggregated EC2 instance metrics such as CPU utilization,disk reads and writes, and network traffic for a given fleet. These metrics enable you to find visibility into the usage and performance of your Spot fleets, or to dynamically scale in response to application load.
Source: amazon.com