Use the over allocation alerts to spot projects and resources with over allocation problems. Then expand the work to investigate further.

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.