Use the over allocation alerts to spot projects and resources with over allocation problems. Then expand the work to investigate further.
Note: This article refers to projects. Depending on your configuration, you may see alternate terminology used in your environment.
Resource view
Resource view shows you which resources are over allocated and when.
Project view
Project view will show you which projects are impacted by the over allocations.
Priority view
Priority view works a little differently. Because work is ranked by priority, it will show you which projects get impacted first by over allocation issues.
If you want to dig a little deeper into how Kelloo decides to set alerts, this is a good place to start.